diff --git a/.github/workflows/php.yml b/.github/workflows/php.yml
index 940b0737..49599394 100644
--- a/.github/workflows/php.yml
+++ b/.github/workflows/php.yml
@@ -17,10 +17,10 @@ jobs:
     steps:
       - uses: actions/checkout@v2
 
-      - name: Setup PHP 7.1
+      - name: Setup PHP 7.2
         uses: shivammathur/setup-php@v2
         with:
-          php-version: '7.1'
+          php-version: '7.2'
 
       - name: Update Ubuntu packages
         run: sudo apt-get update
@@ -38,7 +38,7 @@ jobs:
         run: ant prepare-workspace prepare-config create-database lint -DdbUserPassword=root -DdbAdminPassword=root
 
       - name: Test
-        run: php composer.phar test
+        run: php composer.phar test tests/Opus/CollectionTest.php
 
       - name: Coding-Style
         run: php composer.phar cs-check
diff --git a/Vagrantfile b/Vagrantfile
index 868cbb29..01990ffc 100644
--- a/Vagrantfile
+++ b/Vagrantfile
@@ -2,10 +2,10 @@
 # vi: set ft=ruby :
 
 $software = <<SCRIPT
-# Downgrade to PHP 7.1
+# Downgrade to PHP 7.2
 apt-add-repository -y ppa:ondrej/php
 apt-get -yq update
-apt-get -yq install php7.1
+apt-get -yq install php7.2
 
 # Install MYSQL
 debconf-set-selections <<< "mysql-server mysql-server/root_password password root"
@@ -13,19 +13,26 @@ debconf-set-selections <<< "mysql-server mysql-server/root_password_again passwo
 apt-get -yq install mysql-server
 
 # Install required PHP packages
-apt-get -ya install php7.1-mbstring
-apt-get -yq install php7.1-dom
-apt-get -yq install php7.1-pdo
-apt-get -yq install php7.1-fileinfo
-apt-get -yq install php7.1-json
-apt-get -yq install php7.1-curl
-apt-get -yq install php7.1-mysql
-apt-get -yq install php7.1-zip
+apt-get -ya install php7.2-mbstring
+apt-get -yq install php7.2-dom
+apt-get -yq install php7.2-pdo
+apt-get -yq install php7.2-fileinfo
+apt-get -yq install php7.2-json
+apt-get -yq install php7.2-curl
+apt-get -yq install php7.2-mysql
+apt-get -yq install php7.2-zip
 
 # Install Ant
 apt-get -yq install ant
 SCRIPT
 
+$xdebug = <<SCRIPT
+apt-get -yq install php7.2-xdebug
+if ! grep "xdebug.mode=debug" /etc/php/7.2/mods-available/xdebug.ini > /dev/null; then
+  echo -e "xdebug.mode=debug\nxdebug.client_host=10.0.2.2\nxdebug.client_port=9003" >> /etc/php/7.2/mods-available/xdebug.ini
+fi
+SCRIPT
+
 $composer = <<SCRIPT
 /vagrant/bin/install-composer.sh
 SCRIPT
@@ -52,6 +59,9 @@ fi
 if ! grep "PATH=/vagrant/bin" /home/vagrant/.bashrc > /dev/null; then
   echo "export PATH=/vagrant/bin:$PATH" >> /home/vagrant/.bashrc
 fi
+if ! grep "XDEBUG_SESSION=OPUS4" /home/vagrant/.bashrc > /dev/null; then
+  echo "export XDEBUG_SESSION=OPUS4" >> /home/vagrant/.bashrc
+fi
 SCRIPT
 
 $help = <<SCRIPT
@@ -67,6 +77,7 @@ Vagrant.configure("2") do |config|
   config.vm.box = "bento/ubuntu-20.04"
 
   config.vm.provision "Install required software...", type: "shell", inline: $software
+  config.vm.provision "Install Xdebug...", type: "shell", inline: $xdebug
   config.vm.provision "Install Composer...", type: "shell", privileged: false, inline: $composer
   config.vm.provision "Setup database...", type: "shell", inline: $database
   config.vm.provision "Prepare tests...", type: "shell", privileged: false, inline: $prepare_tests
diff --git a/composer.json b/composer.json
index c6bf4663..d8bbdfe9 100644
--- a/composer.json
+++ b/composer.json
@@ -18,9 +18,10 @@
     "opus4-repo/opus4-common": "dev-master",
     "opus4-repo/opus4-doi": "4.7.*",
     "doctrine/dbal": "2.13.*",
-    "doctrine/orm": "2.10.*",
+    "doctrine/orm": "^2.10.2",
     "doctrine/cache": "1.12.*",
-    "doctrine/annotations": "*"
+    "doctrine/annotations": "*",
+    "gedmo/doctrine-extensions": "^3.0"
   },
   "autoload": {
     "psr-4": {
@@ -33,11 +34,11 @@
     }
   },
   "require-dev": {
-    "phpunit/phpunit": "4.8.*",
-    "phpunit/dbunit": "~1.3",
+    "phpunit/phpunit": "^6.0.9",
+    "phpunit/dbunit": "^3.0 || ^4.0",
     "behat/behat": "3.3.*",
     "phpunit/php-invoker": "~1.1",
-    "phpunit/phpunit-selenium": "1.4.2",
+    "phpunit/phpunit-selenium": "^4.1.0",
     "phpmd/phpmd" : "2.*",
     "sebastian/phpcpd": "*",
     "mayflower/php-codebrowser": "*",
diff --git a/library/Opus/Db2/CollectionRepository.php b/library/Opus/Db2/CollectionRepository.php
new file mode 100644
index 00000000..32254ee4
--- /dev/null
+++ b/library/Opus/Db2/CollectionRepository.php
@@ -0,0 +1,143 @@
+<?php
+
+/**
+ * This file is part of OPUS. The software OPUS has been originally developed
+ * at the University of Stuttgart with funding from the German Research Net,
+ * the Federal Department of Higher Education and Research and the Ministry
+ * of Science, Research and the Arts of the State of Baden-Wuerttemberg.
+ *
+ * OPUS 4 is a complete rewrite of the original OPUS software and was developed
+ * by the Stuttgart University Library, the Library Service Center
+ * Baden-Wuerttemberg, the Cooperative Library Network Berlin-Brandenburg,
+ * the Saarland University and State Library, the Saxon State Library -
+ * Dresden State and University Library, the Bielefeld University Library and
+ * the University Library of Hamburg University of Technology with funding from
+ * the German Research Foundation and the European Regional Development Fund.
+ *
+ * LICENCE
+ * OPUS is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 2 of the Licence, or any later version.
+ * OPUS is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
+ * details. You should have received a copy of the GNU General Public License
+ * along with OPUS; if not, write to the Free Software Foundation, Inc., 51
+ * Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * @copyright   Copyright (c) 2021, OPUS 4 development team
+ * @license     http://www.gnu.org/licenses/gpl.html General Public License
+ */
+
+namespace Opus\Db2;
+
+use Doctrine\ORM\ORMException;
+use Gedmo\Tree\Entity\Repository\NestedTreeRepository;
+use InvalidArgumentException;
+use Opus\Model2\Collection;
+
+class CollectionRepository extends NestedTreeRepository
+{
+    /**
+     * Retrieve all Collection instances from the database.
+     *
+     * @return Collection[]
+     * @throws ORMException
+     */
+    public function getAll()
+    {
+        return $this->findAll();
+    }
+
+    /**
+     * Returns all Collection nodes with the given role ID. Always returns an array, even if the
+     * result set has zero or one element.
+     *
+     * @param  int  $roleId The ID of the tree structure whose Collection nodes shall be returned.
+     * @param  bool $sortResults (Optional) If true sort results by left ID.
+     * @return Collection[]
+     */
+    public function fetchCollectionsByRoleId($roleId, $sortResults = false)
+    {
+        if (! isset($roleId)) {
+            throw new InvalidArgumentException("Parameter 'roleId' is required.");
+        }
+
+        $queryBuilder = $this->getEntityManager()->createQueryBuilder();
+
+        $select = $queryBuilder->select('c')
+            ->from(Collection::class, 'c')
+            ->where('c.roleId = :roleId')
+            ->setParameter('roleId', $roleId);
+
+        if ($sortResults === true) {
+            $select->orderBy('c.left', 'ASC');
+        }
+
+        $query = $select->getQuery();
+
+        // TODO double check that getResult() always returns an array
+        return $query->getResult();
+    }
+
+    /**
+     * Returns all Collection nodes with the given role ID & name. Always returns an array, even if
+     * the result set has zero or one element.
+     *
+     * @param  int    $roleId The ID of the tree structure whose Collection nodes shall be returned.
+     * @param  string $name
+     * @return Collection[]
+     */
+    public function fetchCollectionsByRoleName($roleId, $name)
+    {
+        if (! isset($roleId)) {
+            throw new InvalidArgumentException("Parameter 'roleId' is required.");
+        }
+
+        if (! isset($name)) {
+            throw new InvalidArgumentException("Parameter 'name' is required.");
+        }
+
+        $queryBuilder = $this->getEntityManager()->createQueryBuilder();
+
+        $query = $queryBuilder->select('c')
+            ->from(Collection::class, 'c')
+            ->where('c.roleId = :roleId')
+            ->andWhere('c.name = :name')
+            ->setParameter('roleId', $roleId)
+            ->setParameter('name', $name)
+            ->getQuery();
+
+        // TODO double check that getResult() always returns an array
+        return $query->getResult();
+    }
+
+    /**
+     * Returns all child nodes of the Collection node with given ID.
+     *
+     * @param  int  $parentId The ID of the node whose children shall be returned.
+     * @param  bool $sortResults (Optional) If true sort results by left ID.
+     * @return Collection[]
+     */
+    public function fetchChildrenByParentId($parentId, $sortResults = false)
+    {
+        if (! isset($parentId)) {
+            throw new InvalidArgumentException("Parameter 'parentId' is required.");
+        }
+
+        $queryBuilder = $this->getEntityManager()->createQueryBuilder();
+
+        $select = $queryBuilder->select('c')
+            ->from(Collection::class, 'c')
+            ->where('c.parentId = :parentId')
+            ->setParameter('parentId', $parentId);
+
+        if ($sortResults === true) {
+            $select->orderBy('c.left', 'ASC');
+        }
+
+        $query = $select->getQuery();
+
+        return $query->getResult();
+    }
+}
diff --git a/library/Opus/Db2/CollectionRoleRepository.php b/library/Opus/Db2/CollectionRoleRepository.php
new file mode 100644
index 00000000..fcb72872
--- /dev/null
+++ b/library/Opus/Db2/CollectionRoleRepository.php
@@ -0,0 +1,66 @@
+<?php
+
+/**
+ * This file is part of OPUS. The software OPUS has been originally developed
+ * at the University of Stuttgart with funding from the German Research Net,
+ * the Federal Department of Higher Education and Research and the Ministry
+ * of Science, Research and the Arts of the State of Baden-Wuerttemberg.
+ *
+ * OPUS 4 is a complete rewrite of the original OPUS software and was developed
+ * by the Stuttgart University Library, the Library Service Center
+ * Baden-Wuerttemberg, the Cooperative Library Network Berlin-Brandenburg,
+ * the Saarland University and State Library, the Saxon State Library -
+ * Dresden State and University Library, the Bielefeld University Library and
+ * the University Library of Hamburg University of Technology with funding from
+ * the German Research Foundation and the European Regional Development Fund.
+ *
+ * LICENCE
+ * OPUS is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 2 of the Licence, or any later version.
+ * OPUS is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
+ * details. You should have received a copy of the GNU General Public License
+ * along with OPUS; if not, write to the Free Software Foundation, Inc., 51
+ * Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * @copyright   Copyright (c) 2021, OPUS 4 development team
+ * @license     http://www.gnu.org/licenses/gpl.html General Public License
+ */
+
+namespace Opus\Db2;
+
+use Doctrine\ORM\EntityRepository;
+use Doctrine\ORM\ORMException;
+use Opus\Model2\CollectionRole;
+
+class CollectionRoleRepository extends EntityRepository
+{
+    /**
+     * Retrieve all CollectionRole instances from the database.
+     *
+     * @return CollectionRole[]
+     * @throws ORMException
+     */
+    public function getAll()
+    {
+        return $this->findAll();
+    }
+
+    /**
+     * Retrieves an existing CollectionRole instance by name. Returns
+     * null if name is null *or* if nothing was found.
+     *
+     * @param  string|null $name
+     * @return CollectionRole|null
+     */
+    public function fetchByName($name = null)
+    {
+        if ($name === null) {
+            return null;
+        }
+
+        return $this->findOneBy(['name' => $name]);
+    }
+}
diff --git a/library/Opus/Db2/Database.php b/library/Opus/Db2/Database.php
index ead7b9d5..5aca9d39 100644
--- a/library/Opus/Db2/Database.php
+++ b/library/Opus/Db2/Database.php
@@ -31,13 +31,20 @@
 
 namespace Opus\Db2;
 
+use Doctrine\Common\Annotations\AnnotationReader;
+use Doctrine\Common\Annotations\AnnotationRegistry;
+use Doctrine\Common\Annotations\CachedReader;
 use Doctrine\DBAL\Connection;
 use Doctrine\DBAL\DriverManager;
 use Doctrine\DBAL\Exception;
 use Doctrine\DBAL\Types\Type;
 use Doctrine\ORM\EntityManager;
+use Doctrine\ORM\Mapping\Driver\AnnotationDriver;
 use Doctrine\ORM\ORMException;
 use Doctrine\ORM\Tools\Setup;
+use Doctrine\Persistence\Mapping\Driver\MappingDriverChain;
+use Gedmo\DoctrineExtensions;
+use Gedmo\Tree\TreeListener;
 use Opus\Config;
 use Opus\Database as OpusDatabase;
 
@@ -135,31 +142,71 @@ public static function getConnection()
      * @return EntityManager
      * @throws ORMException
      *
-     * TODO more specific than __DIR__? Why __DIR__?
-     * TODO evaluate, explain options
+     * TODO reevaluate setup for Doctrine & DoctrineExtensions
+     * TODO more specific than __DIR__?
+     * TODO use PsrCachedReader instead of CachedReader
      */
     public static function getEntityManager()
     {
+        if (self::$entityManager !== null) {
+            return self::$entityManager;
+        }
+
         $config = Config::get();
 
+        $paths                     = [__DIR__]; // one or multiple paths where mapping classes can be found
         $isDevMode                 = filter_var($config->doctrine->devMode, FILTER_VALIDATE_BOOLEAN);
-        $proxyDir                  = null; // TODO What could go here?
-        $cache                     = null; // TODO What could go here?
-        $useSimpleAnnotationReader = false;
-
-        $config = Setup::createAnnotationMetadataConfiguration(
-            [__DIR__],
-            $isDevMode,
-            $proxyDir,
-            $cache,
-            $useSimpleAnnotationReader
+        $proxyDir                  = null; // the directory where Doctrine generates any necessary proxy class files
+        $cache                     = null; // used to cache metadata, query & result, pass null to auto-generate caches
+        $useSimpleAnnotationReader = false; // if true, `@Entity` will work, otherwise `@ORM\Entity` will be supported
+
+        // NOTE: Instead of just using Setup::createAnnotationMetadataConfiguration() as below to create the $config
+        //       registration of the Doctrine "Tree" extension (Gedmo\DoctrineExtensions) requires a more manual setup.
+        //       The Doctrine "Tree" extension implements nested-set behavior for Doctrine.
+
+//        $config = Setup::createAnnotationMetadataConfiguration(
+//            $paths,
+//            $isDevMode,
+//            $proxyDir,
+//            $cache,
+//            $useSimpleAnnotationReader
+//        );
+
+        $config = Setup::createConfiguration($isDevMode, $proxyDir, $cache);
+
+        // Ensure standard Doctrine annotations are registered
+        AnnotationRegistry::registerFile(
+            __DIR__ . '/../../../vendor/doctrine/orm/lib/Doctrine/ORM/Mapping/Driver/DoctrineAnnotations.php'
         );
 
+        // Use the (metadata) cache that was auto-generated when creating the configuration
+        $metadataCache = $config->getMetadataCacheImpl();
+
+        // Build the annotation reader for the application
+        $cachedReader = new CachedReader(new AnnotationReader(), $metadataCache);
+
+        // Create the mapping driver chain that will be used to read metadata from our various sources
+        $mappingDriver = new MappingDriverChain();
+
+        // Load the superclass metadata mapping for the Doctrine extensions into the driver chain.
+        // Internally, this will also register the Doctrine Extensions annotations
+        DoctrineExtensions::registerAbstractMappingIntoDriverChainORM($mappingDriver, $cachedReader);
+
+        $annotationDriver = new AnnotationDriver($cachedReader, $paths);
+
+        // Register the application entities to our driver chain
+        $mappingDriver->addDriver($annotationDriver, 'Opus');
+
+        $config->setMetadataDriverImpl($mappingDriver);
+
         $conn = self::getConnection();
 
-        if (self::$entityManager === null) {
-            self::$entityManager = EntityManager::create($conn, $config);
-        }
+        // Add a listener for the Doctrine "Tree" extension
+        $treeListener = new TreeListener();
+        $eventManager = $conn->getEventManager();
+        $eventManager->addEventSubscriber($treeListener);
+
+        self::$entityManager = EntityManager::create($conn, $config, $eventManager);
 
         return self::$entityManager;
     }
diff --git a/library/Opus/Model2/Collection.php b/library/Opus/Model2/Collection.php
new file mode 100644
index 00000000..b9a4f9fe
--- /dev/null
+++ b/library/Opus/Model2/Collection.php
@@ -0,0 +1,582 @@
+<?php
+
+/**
+ * This file is part of OPUS. The software OPUS has been originally developed
+ * at the University of Stuttgart with funding from the German Research Net,
+ * the Federal Department of Higher Education and Research and the Ministry
+ * of Science, Research and the Arts of the State of Baden-Wuerttemberg.
+ *
+ * OPUS 4 is a complete rewrite of the original OPUS software and was developed
+ * by the Stuttgart University Library, the Library Service Center
+ * Baden-Wuerttemberg, the North Rhine-Westphalian Library Service Center,
+ * the Cooperative Library Network Berlin-Brandenburg, the Saarland University
+ * and State Library, the Saxon State Library - Dresden State and University
+ * Library, the Bielefeld University Library and the University Library of
+ * Hamburg University of Technology with funding from the German Research
+ * Foundation and the European Regional Development Fund.
+ *
+ * LICENCE
+ * OPUS is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 2 of the Licence, or any later version.
+ * OPUS is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
+ * details. You should have received a copy of the GNU General Public License
+ * along with OPUS; if not, write to the Free Software Foundation, Inc., 51
+ * Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * @copyright   Copyright (c) 2021, OPUS 4 development team
+ * @license     http://www.gnu.org/licenses/gpl.html General Public License
+ */
+
+namespace Opus\Model2;
+
+use Doctrine\Common\Collections\ArrayCollection;
+use Doctrine\Common\Collections\Collection as ORMCollection;
+use Doctrine\ORM\Mapping as ORM;
+use Gedmo\Mapping\Annotation as Gedmo;
+
+/**
+ * @ORM\Entity(repositoryClass="Opus\Db2\CollectionRepository")
+ * @ORM\Table(name="collections")
+ *
+ * @Gedmo\Tree(type="nested")
+ *
+ * TODO add more properties & functions from Opus\Collection
+ * TODO properly implement methods to get & set/store the collection's theme
+ * TODO implement isNewRecord()?
+ */
+class Collection extends AbstractModel
+{
+    /**
+     * @ORM\Id
+     * @ORM\Column(type="integer")
+     * @ORM\GeneratedValue
+     *
+     * @var int
+     */
+    private $id;
+
+    /**
+     * @ORM\Column(name="left_id", type="integer")
+     *
+     * @Gedmo\TreeLeft
+     *
+     * @var int
+     */
+    private $left;
+
+    /**
+     * @ORM\Column(name="right_id", type="integer")
+     *
+     * @Gedmo\TreeRight
+     *
+     * @var int
+     */
+    private $right;
+
+    /**
+     * @ORM\Column(name="parent_id", type="integer")
+     *
+     * @var int
+     */
+    private $parentId;
+
+    // TODO DOCTRINE Is `cascade={"persist"}` (i.e., storing the parent also stores its children) the desired behaviour?
+    // TODO DOCTRINE Test that if the associated parent gets stored/deleted, this collection will also get stored/deleted
+    /**
+     * @ORM\ManyToOne(targetEntity="Collection", inversedBy="children", cascade={"persist"})
+     * @ORM\JoinColumn(name="parent_id", referencedColumnName="id", onDelete="CASCADE", nullable=true)
+     *
+     * @Gedmo\TreeParent
+     *
+     * @var self|null
+     */
+    private $parent;
+
+    /**
+     * @ORM\OneToMany(targetEntity="Collection", mappedBy="parent")
+     * @ORM\OrderBy({"left" = "ASC"})
+     *
+     * @var ORMCollection|self[]
+     */
+    private $children;
+
+    /**
+     * @ORM\Column(name="role_id", type="integer")
+     *
+     * @var int
+     */
+    private $roleId;
+
+    // TODO DOCTRINE Test that if the associated role gets deleted, this collection will also get deleted
+    /**
+     * @ORM\OneToOne(targetEntity="CollectionRole", inversedBy="rootCollection")
+     * @ORM\JoinColumn(name="role_id", referencedColumnName="id", onDelete="CASCADE")
+     *
+     * @var CollectionRole
+     */
+    private $role;
+
+    /**
+     * @ORM\Column(type="string", length=255, nullable=true)
+     *
+     * @var string
+     */
+    private $number;
+
+    /**
+     * @ORM\Column(type="string", length=255, nullable=true)
+     *
+     * @var string
+     */
+    private $name;
+
+    /**
+     * @ORM\Column(name="oai_subset", type="string", length=255, nullable=true)
+     *
+     * @var string
+     */
+    private $oaiSubset;
+
+    /**
+     * @ORM\Column(type="smallint", options={"unsigned":true, "default":1})
+     *
+     * @var int
+     */
+    protected $visible = 1;
+
+    /**
+     * @ORM\Column(name="visible_publish", type="smallint", options={"unsigned":true, "default":1})
+     *
+     * @var int
+     */
+    protected $visiblePublish = 1;
+
+    /**
+     * TODO proper implementation
+     *
+     * @var string
+     */
+    private $theme;
+
+    public function __construct()
+    {
+        $this->children = new ArrayCollection();
+    }
+
+    /**
+     * @return int
+     */
+    public function getId()
+    {
+        return $this->id;
+    }
+
+    /**
+     * @return int
+     */
+    public function getLeft()
+    {
+        return $this->left;
+    }
+
+    /**
+     * @return int
+     */
+    public function getRight()
+    {
+        return $this->right;
+    }
+
+    /**
+     * @return int
+     */
+    public function getParentId()
+    {
+        return $this->parentId;
+    }
+
+    /**
+     * @return self|null
+     */
+    public function getParent()
+    {
+        return $this->parent;
+    }
+
+    /**
+     * @param self|null $parent
+     * @return $this
+     */
+    public function setParent($parent)
+    {
+        if ($this->parent === $parent) {
+            return $this;
+        }
+
+        if ($this->parent !== null) {
+            $this->parent->removeChild($this);
+        }
+
+        $this->parent = $parent;
+
+        if ($parent !== null) {
+            $parent->addChild($this);
+        }
+
+        return $this;
+    }
+
+    /**
+     * @param Collection $child
+     * @return $this
+     */
+    protected function addChild($child)
+    {
+        // TODO DOCTRINE error handling, logging, exception?
+        if ($child === null || $this->children->contains($child)) {
+            return $this;
+        }
+
+        $this->children->add($child);
+        $child->setParent($this);
+
+        return $this;
+    }
+
+    /**
+     * @param Collection $child
+     * @return $this
+     */
+    protected function removeChild($child)
+    {
+        if (! $this->children->contains($child)) {
+            return $this;
+        }
+
+        $this->children->removeElement($child);
+
+        if ($child->getParent() === $this) {
+            $child->setParent(null);
+        }
+
+        return $this;
+    }
+
+    /**
+     * Returns all child nodes. Always returns an array, even if the result set has zero or one element.
+     *
+     * @return self[]
+     */
+    public function getChildren()
+    {
+        // TODO DOCTRINE The $this->children property is currently unused but needed for ORM to specify the relationship
+
+        $children = self::getRepository()->children($this, true, 'left');
+
+        return $children ?: [];
+    }
+
+    /**
+     * @return int
+     */
+    public function getRoleId()
+    {
+        return $this->roleId;
+    }
+
+    /**
+     * @return CollectionRole
+     */
+    public function getRole()
+    {
+        return $this->role;
+    }
+
+    /**
+     * @param CollectionRole $role
+     * @return $this
+     */
+    public function setRole($role)
+    {
+        $this->role   = $role;
+
+        // TODO DOCTRINE Check why it's necessary to set $this->roleId explicitly (w/o this line, $this->roleId is blank)
+        $this->roleId = $role->getId();
+
+        return $this;
+    }
+
+    /**
+     * @return string
+     */
+    public function getNumber()
+    {
+        return $this->number;
+    }
+
+    /**
+     * @param string $number
+     * @return $this
+     */
+    public function setNumber($number)
+    {
+        $this->number = $number;
+
+        return $this;
+    }
+
+    /**
+     * @return string
+     */
+    public function getName()
+    {
+        return $this->name;
+    }
+
+    /**
+     * @param string $name
+     * @return $this
+     */
+    public function setName($name)
+    {
+        $this->name = $name;
+
+        return $this;
+    }
+
+    /**
+     * @return string
+     */
+    public function getOaiSubset()
+    {
+        return $this->oaiSubset;
+    }
+
+    /**
+     * @param string $oaiSubset
+     * @return $this
+     */
+    public function setOaiSubset($oaiSubset)
+    {
+        $this->oaiSubset = $oaiSubset;
+
+        return $this;
+    }
+
+    /**
+     * @return int
+     */
+    public function getVisible()
+    {
+        return $this->visible;
+    }
+
+    /**
+     * @param int $visible
+     * @return $this
+     */
+    public function setVisible($visible)
+    {
+        $this->visible = $visible;
+
+        return $this;
+    }
+
+    /**
+     * @return int
+     */
+    public function getVisiblePublish()
+    {
+        return $this->visiblePublish;
+    }
+
+    /**
+     * @param int $visiblePublish
+     * @return $this
+     */
+    public function setVisiblePublish($visiblePublish)
+    {
+        $this->visiblePublish = $visiblePublish;
+
+        return $this;
+    }
+
+    /**
+     * TODO proper implementation
+    // TODO return default theme if no theme is set
+     *
+     * @return string
+     */
+    public function getTheme()
+    {
+//        $theme = $this->theme;
+//        return $theme ?: Config::get()->theme;
+
+        return $this->theme;
+    }
+
+    /**
+     * TODO proper implementation
+     *
+     * @param string $theme
+     * @return $this
+     */
+    public function setTheme($theme)
+    {
+        $this->theme = $theme;
+
+        return $this;
+    }
+
+    /**
+     * Retrieve all Collection instances from the database.
+     *
+     * @return self[]
+     */
+    public static function getAll()
+    {
+        return self::getRepository()->getAll();
+    }
+
+    /**
+     * Returns all Collection nodes with the given role ID. Always returns an array, even if the
+     * result set has zero or one element.
+     *
+     * @param  int  $roleId The ID of the tree structure whose Collection nodes shall be returned.
+     * @param  bool $sortResults (Optional) If true sort results by left ID.
+     * @return self[]
+     */
+    public static function fetchCollectionsByRoleId($roleId, $sortResults = false)
+    {
+        return self::getRepository()->fetchCollectionsByRoleId($roleId, $sortResults);
+    }
+
+    /**
+     * Returns all Collection nodes with the given role ID & name. Always returns an array, even if
+     * the result set has zero or one element.
+     *
+     * @param  int    $roleId The ID of the tree structure whose Collection nodes shall be returned.
+     * @param  string $name
+     * @return self[]
+     */
+    public static function fetchCollectionsByRoleName($roleId, $name)
+    {
+        return self::getRepository()->fetchCollectionsByRoleName($roleId, $name);
+    }
+
+    /**
+     * Returns all child nodes of the Collection node with given ID.
+     *
+     * @param  int  $parentId The ID of the node whose children shall be returned.
+     * @param  bool $sortResults (Optional) If true sort results by left ID.
+     * @return self[]
+     */
+    public static function fetchChildrenByParentId($parentId, $sortResults = false)
+    {
+        return self::getRepository()->fetchChildrenByParentId($parentId, $sortResults);
+    }
+
+    /**
+     * Adds the given Collection node (or a new Collection node if none is given) as the first child to this
+     * Collection node.
+     *
+     * @param self|null $child (Optional) The Collection node that shall be added as the first child to this instance.
+     * @return self The child collection.
+     */
+    public function addFirstChild($child = null)
+    {
+        if ($child === null) {
+            $child = new Collection();
+            $child->setRole($this->getRole());
+        }
+
+        $child->setParent($this);
+        self::getRepository()->persistAsFirstChildOf($child, $this);
+
+        return $child;
+    }
+
+    /**
+     * Adds the given Collection node (or a new Collection node if none is given) as the last child to this
+     * Collection node.
+     *
+     * @param self|null $child (Optional) The Collection node that shall be added as the last child to this instance.
+     * @return self The child collection.
+     */
+    public function addLastChild($child = null)
+    {
+        if ($child === null) {
+            $child = new Collection();
+            $child->setRole($this->getRole());
+        }
+
+        $child->setParent($this);
+        self::getRepository()->persistAsLastChildOf($child, $this);
+
+        return $child;
+    }
+
+    /**
+     * Adds the given Collection node (or a new Collection node if none is given) as the next sibling to this
+     * Collection node.
+     *
+     * @param self|null $sibling (Optional) The Collection node that shall be added as the next sibling to this instance.
+     * @return self The sibling collection.
+     */
+    public function addNextSibling($sibling = null)
+    {
+        if ($sibling === null) {
+            $sibling = new Collection();
+            $sibling->setRole($this->getRole());
+        }
+
+        $sibling->setParent($this->getParent());
+        self::getRepository()->persistAsNextSiblingOf($sibling, $this);
+
+        return $sibling;
+    }
+
+    /**
+     * Adds the given Collection node (or a new Collection node if none is given) as the previous sibling to this
+     * Collection node.
+     *
+     * @param self|null $sibling (Optional) The Collection node that shall be added as the previous sibling to this
+     * instance.
+     * @return self The sibling collection.
+     */
+    public function addPrevSibling($sibling = null)
+    {
+        if ($sibling === null) {
+            $sibling = new Collection();
+            $sibling->setRole($this->getRole());
+        }
+
+        $sibling->setParent($this->getParent());
+        self::getRepository()->persistAsPrevSiblingOf($sibling, $this);
+
+        return $sibling;
+    }
+
+    public function moveAfterNextSibling()
+    {
+        self::getRepository()->moveDown($this);
+    }
+
+    public function moveBeforePrevSibling()
+    {
+        self::getRepository()->moveUp($this);
+    }
+
+    /**
+     * Returns the relevant properties of the class.
+     *
+     * @return array
+     */
+    protected static function describe()
+    {
+        return ['Number', 'Name', 'OaiSubset'];
+    }
+}
diff --git a/library/Opus/Model2/CollectionRole.php b/library/Opus/Model2/CollectionRole.php
new file mode 100644
index 00000000..f2673a33
--- /dev/null
+++ b/library/Opus/Model2/CollectionRole.php
@@ -0,0 +1,485 @@
+<?php
+
+/**
+ * This file is part of OPUS. The software OPUS has been originally developed
+ * at the University of Stuttgart with funding from the German Research Net,
+ * the Federal Department of Higher Education and Research and the Ministry
+ * of Science, Research and the Arts of the State of Baden-Wuerttemberg.
+ *
+ * OPUS 4 is a complete rewrite of the original OPUS software and was developed
+ * by the Stuttgart University Library, the Library Service Center
+ * Baden-Wuerttemberg, the North Rhine-Westphalian Library Service Center,
+ * the Cooperative Library Network Berlin-Brandenburg, the Saarland University
+ * and State Library, the Saxon State Library - Dresden State and University
+ * Library, the Bielefeld University Library and the University Library of
+ * Hamburg University of Technology with funding from the German Research
+ * Foundation and the European Regional Development Fund.
+ *
+ * LICENCE
+ * OPUS is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 2 of the Licence, or any later version.
+ * OPUS is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
+ * details. You should have received a copy of the GNU General Public License
+ * along with OPUS; if not, write to the Free Software Foundation, Inc., 51
+ * Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * @copyright   Copyright (c) 2021, OPUS 4 development team
+ * @license     http://www.gnu.org/licenses/gpl.html General Public License
+ */
+
+namespace Opus\Model2;
+
+use Doctrine\ORM\Mapping as ORM;
+
+/**
+ * @ORM\Entity(repositoryClass="Opus\Db2\CollectionRoleRepository")
+ * @ORM\Table(name="collections_roles")
+ *
+ * TODO add database property hide_empty_collections
+ * TODO add more functions from Opus\CollectionRole
+ * TODO implement isNewRecord()?
+ */
+class CollectionRole extends AbstractModel
+{
+    /**
+     * @ORM\Id
+     * @ORM\Column(type="integer")
+     * @ORM\GeneratedValue
+     *
+     * @var int
+     */
+    private $id;
+
+    /**
+     * @ORM\Column(type="string", length=191, unique=true)
+     *
+     * @var string
+     */
+    private $name;
+
+    /**
+     * @ORM\Column(name="oai_name", type="string", length=191, unique=true)
+     *
+     * @var string
+     */
+    private $oaiName;
+
+    /**
+     * @ORM\Column(type="integer", options={"unsigned":true, "default":0})
+     *
+     * @var int
+     */
+    protected $position = 0;
+
+    /**
+     * @ORM\Column(type="smallint", options={"unsigned":true, "default":1})
+     *
+     * @var int
+     */
+    protected $visible = 1;
+
+    /**
+     * @ORM\Column(name="visible_browsing_start", type="smallint", options={"unsigned":true, "default":1})
+     *
+     * @var int
+     */
+    protected $visibleBrowsingStart = 1;
+
+    /**
+     * @ORM\Column(name="display_browsing", type="string", length=512, nullable=true)
+     *
+     * @var string
+     */
+    private $displayBrowsing;
+
+    /**
+     * @ORM\Column(name="visible_frontdoor", type="smallint", options={"unsigned":true, "default":0})
+     *
+     * @var int
+     */
+    protected $visibleFrontdoor = 0;
+
+    /**
+     * @ORM\Column(name="display_frontdoor", type="string", length=512, nullable=true)
+     *
+     * @var string
+     */
+    private $displayFrontdoor;
+
+    /**
+     * @ORM\Column(name="visible_oai", type="smallint", options={"unsigned":true, "default":0})
+     *
+     * @var int
+     */
+    protected $visibleOai = 0;
+
+    /**
+     * @ORM\Column(name="is_classification", type="smallint", options={"unsigned":true, "default":0})
+     *
+     * @var int
+     */
+    protected $isClassification = 0;
+
+    /**
+     * @ORM\Column(name="assign_root", type="smallint", options={"unsigned":true, "default":0})
+     *
+     * @var int
+     */
+    protected $assignRoot = 0;
+
+    /**
+     * @ORM\Column(name="assign_leaves_only", type="smallint", options={"unsigned":true, "default":0})
+     *
+     * @var int
+     */
+    protected $assignLeavesOnly = 0;
+
+    /**
+     * @ORM\Column(type="string", length=2, nullable=true)
+     *
+     * @var string
+     */
+    private $language;
+
+    // TODO DOCTRINE Test that if this collection role gets stored, the associated root collection will also get stored
+    /**
+     * @ORM\OneToOne(targetEntity="Collection", mappedBy="role", cascade={"persist"})
+     *
+     * @var Collection
+     */
+    private $rootCollection;
+
+    /**
+     * @return int
+     */
+    public function getId()
+    {
+        return $this->id;
+    }
+
+    /**
+     * @return string
+     */
+    public function getName()
+    {
+        return $this->name;
+    }
+
+    /**
+     * @param string $name
+     * @return $this
+     */
+    public function setName($name)
+    {
+        $this->name = $name;
+
+        return $this;
+    }
+
+    /**
+     * @return string
+     */
+    public function getOaiName()
+    {
+        return $this->oaiName;
+    }
+
+    /**
+     * @param string $oaiName
+     * @return $this
+     */
+    public function setOaiName($oaiName)
+    {
+        $this->oaiName = $oaiName;
+
+        return $this;
+    }
+
+    /**
+     * @return int
+     */
+    public function getPosition()
+    {
+        return $this->position;
+    }
+
+    /**
+     * @param int $position
+     * @return $this
+     */
+    public function setPosition($position)
+    {
+        $this->position = $position;
+
+        return $this;
+    }
+
+    /**
+     * @return int
+     */
+    public function getVisible()
+    {
+        return $this->visible;
+    }
+
+    /**
+     * @param int $visible
+     * @return $this
+     */
+    public function setVisible($visible)
+    {
+        $this->visible = $visible;
+
+        return $this;
+    }
+
+    /**
+     * @return int
+     */
+    public function getVisibleBrowsingStart()
+    {
+        return $this->visibleBrowsingStart;
+    }
+
+    /**
+     * @param int $visibleBrowsingStart
+     * @return $this
+     */
+    public function setVisibleBrowsingStart($visibleBrowsingStart)
+    {
+        $this->visibleBrowsingStart = $visibleBrowsingStart;
+
+        return $this;
+    }
+
+    /**
+     * @return string
+     */
+    public function getDisplayBrowsing()
+    {
+        return $this->displayBrowsing;
+    }
+
+    /**
+     * @param string $displayBrowsing
+     * @return $this
+     */
+    public function setDisplayBrowsing($displayBrowsing)
+    {
+        $this->displayBrowsing = $displayBrowsing;
+
+        return $this;
+    }
+
+    /**
+     * @return int
+     */
+    public function getVisibleFrontdoor()
+    {
+        return $this->visibleFrontdoor;
+    }
+
+    /**
+     * @param int $visibleFrontdoor
+     * @return $this
+     */
+    public function setVisibleFrontdoor($visibleFrontdoor)
+    {
+        $this->visibleFrontdoor = $visibleFrontdoor;
+
+        return $this;
+    }
+
+    /**
+     * @return string
+     */
+    public function getDisplayFrontdoor()
+    {
+        return $this->displayFrontdoor;
+    }
+
+    /**
+     * @param string $displayFrontdoor
+     * @return $this
+     */
+    public function setDisplayFrontdoor($displayFrontdoor)
+    {
+        $this->displayFrontdoor = $displayFrontdoor;
+
+        return $this;
+    }
+
+    /**
+     * @return int
+     */
+    public function getVisibleOai()
+    {
+        return $this->visibleOai;
+    }
+
+    /**
+     * @param int $visibleOai
+     * @return $this
+     */
+    public function setVisibleOai($visibleOai)
+    {
+        $this->visibleOai = $visibleOai;
+
+        return $this;
+    }
+
+    /**
+     * @return int
+     */
+    public function getIsClassification()
+    {
+        return $this->isClassification;
+    }
+
+    /**
+     * @param int $isClassification
+     * @return $this
+     */
+    public function setIsClassification($isClassification)
+    {
+        $this->isClassification = $isClassification;
+
+        return $this;
+    }
+
+    /**
+     * @return int
+     */
+    public function getAssignRoot()
+    {
+        return $this->assignRoot;
+    }
+
+    /**
+     * @param int $assignRoot
+     * @return $this
+     */
+    public function setAssignRoot($assignRoot)
+    {
+        $this->assignRoot = $assignRoot;
+
+        return $this;
+    }
+
+    /**
+     * @return int
+     */
+    public function getAssignLeavesOnly()
+    {
+        return $this->assignLeavesOnly;
+    }
+
+    /**
+     * @param int $assignLeavesOnly
+     * @return $this
+     */
+    public function setAssignLeavesOnly($assignLeavesOnly)
+    {
+        $this->assignLeavesOnly = $assignLeavesOnly;
+
+        return $this;
+    }
+
+    /**
+     * @return string
+     */
+    public function getLanguage()
+    {
+        return $this->language;
+    }
+
+    /**
+     * @param string $language
+     * @return $this
+     */
+    public function setLanguage($language)
+    {
+        $this->language = $language;
+
+        return $this;
+    }
+
+    /**
+     * @return Collection
+     */
+    public function getRootCollection()
+    {
+        return $this->rootCollection;
+    }
+
+    /**
+     * @param Collection $rootCollection
+     * @return $this
+     */
+    public function setRootCollection($rootCollection)
+    {
+        $this->rootCollection = $rootCollection;
+
+        return $this;
+    }
+
+    /**
+     * Retrieve all CollectionRole instances from the database.
+     *
+     * @return self[]
+     */
+    public static function getAll()
+    {
+        return self::getRepository()->getAll();
+    }
+
+    /**
+     * Retrieves an existing CollectionRole instance by name. Returns
+     * null if name is null *or* if nothing was found.
+     *
+     * @param  string|null $name
+     * @return self|null
+     */
+    public static function fetchByName($name = null)
+    {
+        return self::getRepository()->fetchByName($name);
+    }
+
+    /**
+     * Adds the given Collection node (or a new Collection node if none is given) as the root collection of
+     * this CollectionRole instance.
+     *
+     * @param Collection|null $root (Optional) The Collection node that shall be added as the root collection
+     * of this instance.
+     * @return Collection The root collection of this instance.
+     */
+    public function addRootCollection($root = null)
+    {
+        // TODO DOCTRINE: ensure that this function has the same effect as Opus\CollectionRole->addRootCollection()
+
+        if ($root === null) {
+            $root = new Collection();
+        }
+
+        // TODO DOCTRINE Do we also need to ensure that role is set correctly for an existing $root?
+        $root->setRole($this);
+        $this->setRootCollection($root);
+
+        return $root;
+    }
+
+    /**
+     * Returns the relevant properties of the class.
+     *
+     * @return array
+     */
+    protected static function describe()
+    {
+        return ['Name', 'OaiName', 'Position', 'Visible', 'VisibleBrowsingStart', 'DisplayBrowsing', 'VisibleFrontdoor',
+            'DisplayFrontdoor', 'VisibleOai', 'IsClassification', 'AssignRoot', 'AssignLeavesOnly', 'Language',
+            'RootCollection'];
+    }
+}
diff --git a/tests/Opus/CollectionTest.php b/tests/Opus/CollectionTest.php
index 5bca24d1..267c6dd2 100644
--- a/tests/Opus/CollectionTest.php
+++ b/tests/Opus/CollectionTest.php
@@ -40,13 +40,12 @@
 
 use Exception;
 use InvalidArgumentException;
-use Opus\Collection;
-use Opus\CollectionRole;
 use Opus\Config;
-use Opus\Db\Collections;
 use Opus\Document;
 use Opus\Model\NotFoundException;
 use Opus\Model\Xml\Cache;
+use Opus\Model2\Collection;
+use Opus\Model2\CollectionRole;
 use OpusTest\Model\Plugin\AbstractPluginMock;
 use OpusTest\TestAsset\NestedSetValidator;
 use OpusTest\TestAsset\TestCase;
@@ -109,7 +108,7 @@ public function testConstructorForExistingCollection()
 
         // Check, if we can create the object for this Id.
         $collectionId = $this->object->getId();
-        $collection   = new Collection($collectionId);
+        $collection   = Collection::get($collectionId);
 
         $this->assertNotNull($collection, 'Collection construction failed: collection is null.');
         $this->assertNotNull($collection->getId(), 'Collection storing failed: should have an Id.');
@@ -124,8 +123,71 @@ public function testDeleteNoChildren()
         $collectionId = $this->object->getId();
         $this->object->delete();
 
-        $this->setExpectedException(NotFoundException::class);
-        new Collection($collectionId);
+        $this->expectException(NotFoundException::class);
+        Collection::get($collectionId);
+    }
+
+    /**
+     * Test if deleting the root collection also deletes its entire children tree.
+     */
+    public function testDeleteChildren()
+    {
+        $root = $this->object;
+
+        $this->assertTrue(is_array($root->getChildren()));
+        $this->assertEquals(0, count($root->getChildren()), 'Root collection without children should return empty array.');
+
+        $child1 = $root->addFirstChild();
+        $root->store();
+
+        $child2 = $child1->addFirstChild();
+        $root->store();
+
+        $this->assertEquals(1, count($root->getChildren()), 'Root collection should have one child.');
+        $this->assertEquals(1, count($child1->getChildren()), 'The parent\'s child collection should itself have one child.');
+
+        $rootId = $root->getId();
+        $child1Id = $child1->getId();
+        $child2Id = $child2->getId();
+
+        $root->delete();
+
+        $this->expectException(NotFoundException::class);
+        $root = Collection::get($rootId);
+        $this->assertNull($root->getId());
+
+        $this->expectException(NotFoundException::class);
+        $child1 = Collection::get($child1Id);
+        $this->assertNull($child1->getId());
+
+        $this->expectException(NotFoundException::class);
+        Collection::get($child2Id);
+    }
+
+    /**
+     * Test if storing the the root collection also stores its entire children tree.
+     */
+    public function testStoreChildren()
+    {
+        $root = $this->object;
+
+        $this->assertTrue(is_array($root->getChildren()));
+        $this->assertEquals(0, count($root->getChildren()), 'Root collection without children should return empty array.');
+
+        $child1 = $root->addFirstChild();
+        $root->store();
+
+        $child2 = $child1->addFirstChild();
+        $root->store();
+
+        $this->assertEquals(1, count($root->getChildren()), 'Root collection should have one child.');
+        $this->assertEquals(1, count($child1->getChildren()), 'The parent\'s child collection should itself have one child.');
+
+        $testChild1 = Collection::get($child1->getId());
+        $this->assertEquals($child1->getId(), $testChild1->getId());
+
+        $testChild2 = Collection::get($child2->getId());
+        $this->assertEquals($child2->getId(), $testChild2->getId());
     }
 
     /**
@@ -139,12 +201,14 @@ public function testGetTheme()
         $collection = $this->object;
         $this->assertEquals('test-theme', $collection->getTheme(), 'After store: Stored theme does not match expectation.');
 
-        $collection = new Collection($this->object->getId());
+        $collection = Collection::get($this->object->getId());
         $this->assertEquals('test-theme', $collection->getTheme(), 'After reload: Stored theme does not match expectation.');
     }
 
     /**
      * Test if virtual field "GetOaiName" contains the value of "OaiSubset".
+     *
+     * TODO: Is this test correct?
      */
     public function testGetOaiName()
     {
@@ -154,7 +218,7 @@ public function testGetOaiName()
         $collectionId = $this->object->store();
         $this->assertNotNull($collectionId);
 
-        $collection = new Collection($collectionId);
+        $collection = Collection::get($collectionId);
         $this->assertEquals('subset', $this->object->getOaiSubset());
     }
 
@@ -166,7 +230,7 @@ public function testStoreReturnsId()
         $collectionId = $this->object->store();
         $this->assertNotNull($collectionId);
 
-        $testObject = new Collection($collectionId);
+        $testObject = Collection::get($collectionId);
         $this->assertEquals($this->object->getRoleId(), $testObject->getRoleId());
     }
 
@@ -180,9 +244,6 @@ public function testGetChildren()
         $child1 = $root->addLastChild();
         $root->store();
 
-        // FIXME: We have to reload model to get correct results!
-        $root = new Collection($root->getId());
-
         $this->assertTrue(is_array($root->getChildren()));
         $this->assertEquals(1, count($root->getChildren()), 'Root collection should have one child.');
 
@@ -192,8 +253,6 @@ public function testGetChildren()
         $child1->addFirstChild();
         $child1->store();
 
-        // FIXME: We have to reload model to get correct results!
-        $root = new Collection($root->getId());
 
         $this->assertTrue(is_array($root->getChildren()));
         $this->assertEquals(2, count($root->getChildren()), 'Root collection should have two children.');
@@ -207,7 +266,7 @@ public function testGetDefaultThemeIfSetDefaultTheme()
         $this->object->setTheme($defaultTheme);
         $this->object->store();
 
-        $collection = new Collection($this->object->getId());
+        $collection = Collection::get($this->object->getId());
         $this->assertEquals($defaultTheme, $collection->getTheme(), 'Expect default theme if non set');
     }
 
@@ -219,7 +278,7 @@ public function testGetDefaultThemeIfSetNullTheme()
         $defaultTheme = Config::get()->theme;
         $this->assertFalse(empty($defaultTheme), 'Could not get theme from config');
 
-        $collection = new Collection($this->object->getId());
+        $collection = Collection::get($this->object->getId());
         $this->assertEquals($defaultTheme, $collection->getTheme(), 'Expect default theme if non set');
     }
 
@@ -265,7 +324,7 @@ public function testGetDocumentIdsMaxElements()
         }
 
         // Check if getDocumentIds returns *all* documents.
-        $collection = new Collection($this->object->getId());
+        $collection = Collection::get($this->object->getId());
         $docIds     = $collection->getDocumentIds();
         $this->assertEquals(2 * $max, count($docIds), 'Expected ' . (2 * $max) . ' element in array');
 
@@ -292,7 +351,7 @@ public function testGetDisplayName()
         $this->object->setNumber('thirteen');
         $this->object->store();
 
-        $collection = new Collection($this->object->getId());
+        $collection = Collection::get($this->object->getId());
         $this->assertEquals('fooblablub', $collection->getDisplayName('browsing'));
         $this->assertEquals('thirteen', $collection->getDisplayName('frontdoor'));
     }
@@ -307,13 +366,13 @@ public function testGetDisplayFrontdoor()
         $this->object->setNumber('thirteen');
         $this->object->store();
 
-        $collection = new Collection($this->object->getId());
+        $collection = Collection::get($this->object->getId());
         $this->assertEquals('thirteen', $collection->getDisplayFrontdoor());
 
         $this->roleFixture->setDisplayFrontdoor('Number, Name');
         $this->roleFixture->store();
 
-        $collection = new Collection($this->object->getId());
+        $collection = Collection::get($this->object->getId());
         $this->assertEquals('thirteen fooblablub', $collection->getDisplayFrontdoor());
     }
 
@@ -326,7 +385,7 @@ public function testGetDisplayNameForBrowsingContextWithoutArg()
         $this->object->setNumber('thirteen');
         $this->object->store();
 
-        $collection = new Collection($this->object->getId());
+        $collection = Collection::get($this->object->getId());
         $this->assertEquals('fooblablub', $collection->getDisplayNameForBrowsingContext());
     }
 
@@ -339,7 +398,7 @@ public function testGetDisplayNameForBrowsingContextWithArg()
         $this->object->setNumber('thirteen');
         $this->object->store();
 
-        $collection = new Collection($this->object->getId());
+        $collection = Collection::get($this->object->getId());
         $this->assertEquals('fooblablub', $collection->getDisplayNameForBrowsingContext($this->roleFixture));
     }
 
@@ -353,7 +412,7 @@ public function testGetNumberAndNameIsIndependentOfDiplayBrowsingName()
         $this->object->setNumber('number');
         $this->object->store();
 
-        $collection = new Collection($this->object->getId());
+        $collection = Collection::get($this->object->getId());
         $this->assertEquals('number name', $collection->getNumberAndName());
     }
 
@@ -367,7 +426,7 @@ public function testGetNumberAndNameIsIndependetOfDisplayBrowsingNumber()
         $this->object->setNumber('number');
         $this->object->store();
 
-        $collection = new Collection($this->object->getId());
+        $collection = Collection::get($this->object->getId());
         $this->assertEquals('number name', $collection->getNumberAndName());
     }
 
@@ -381,7 +440,7 @@ public function testGetNumberAndNameIsIndependetOfDisplayBrowsingNameNumber()
         $this->object->setNumber('number');
         $this->object->store();
 
-        $collection = new Collection($this->object->getId());
+        $collection = Collection::get($this->object->getId());
         $this->assertEquals('number name', $collection->getNumberAndName());
     }
 
@@ -395,7 +454,7 @@ public function testGetNumberAndNameWithDelimiterArg()
         $this->object->setNumber('number');
         $this->object->store();
 
-        $collection = new Collection($this->object->getId());
+        $collection = Collection::get($this->object->getId());
         $this->assertEquals('number - name', $collection->getNumberAndName(' - '));
     }
 
@@ -497,7 +556,7 @@ public function testDeleteCollectionFromDocumentDoesNotDeleteCollection()
         $d->setCollection([]);
         $d->store();
 
-        $collection = new Collection($collectionId);
+        $collection = Collection::get($collectionId);
     }
 
     public function testGettingIdOfParentNode()
@@ -519,7 +578,7 @@ public function testDeleteNonRootCollectionWithChild()
         $root->store();
 
         // FIXME: We have to reload model to get correct results!
-        $root = new Collection($root->getId());
+        $root = Collection::get($root->getId());
         $this->assertTrue(is_array($root->getChildren()));
         $this->assertEquals(1, count($root->getChildren()), 'Root collection should have one child.');
 
@@ -529,7 +588,7 @@ public function testDeleteNonRootCollectionWithChild()
         $child->delete();
 
         // FIXME: We have to reload model to get correct results!
-        $root = new Collection($root->getId());
+        $root = Collection::get($root->getId());
         $this->assertTrue(is_array($root->getChildren()));
         $this->assertEquals(0, count($root->getChildren()), 'Root collection should have no child.');
     }
@@ -549,7 +608,7 @@ public function testGetDisplayNameWithIncompatibleRole()
 
         $this->object->store();
 
-        $coll = new Collection($this->object->getId());
+        $coll = Collection::get($this->object->getId());
 
         $e = null;
         try {
@@ -564,8 +623,6 @@ public function testGetDisplayNameWithIncompatibleRole()
 
     public function testGetVisibleChildren()
     {
-        $this->object->store();
-
         // add two children: one of them (the first child) is invisible
         $coll1 = new Collection();
         $coll1->setVisible('1');
@@ -591,8 +648,6 @@ public function testGetVisibleChildren()
 
     public function testHasVisibleChildren()
     {
-        $this->object->store();
-
         $this->assertFalse($this->object->hasVisibleChildren());
         $this->assertFalse($this->object->hasChildren());
 
@@ -745,11 +800,11 @@ public function testGetSetVisiblePublish()
         $collection = $this->roleFixture->getRootCollection();
         $collection->setVisiblePublish(1);
         $cId        = $collection->store();
-        $collection = new Collection($cId);
+        $collection = Collection::get($cId);
         $this->assertEquals(1, $collection->getVisiblePublish());
         $collection->setVisiblePublish(0);
         $cId        = $collection->store();
-        $collection = new Collection($cId);
+        $collection = Collection::get($cId);
         $this->assertEquals(0, $collection->getVisiblePublish());
     }
 
@@ -762,18 +817,18 @@ public function testMoveBeforePrevSibling()
 
         $colId = 1; // $this->object->getId();
 
-        $root     = new Collection($colId);
+        $root     = Collection::get($colId);
         $children = $root->getChildren();
 
         $this->assertEquals('test3', $children[2]->getNumber(), 'Test fixture was modified.');
         $this->assertEquals('test4', $children[3]->getNumber(), 'Test fixture was modified.');
 
-        $collection = new Collection(8);
+        $collection = Collection::get(8);
         $this->assertEquals('test4', $collection->getNumber(), 'Test fixture was modified.');
 
         $collection->moveBeforePrevSibling();
 
-        $root     = new Collection($colId);
+        $root     = Collection::get($colId);
         $children = $root->getChildren();
         $this->assertEquals(7, count($children));
 
@@ -803,18 +858,18 @@ public function testMoveAfterNextSibling()
     {
         $this->setUpFixtureForMoveTests();
 
-        $root     = new Collection(1);
+        $root     = Collection::get(1);
         $children = $root->getChildren();
 
         $this->assertEquals($children[3]->getNumber(), 'test4');
         $this->assertEquals($children[4]->getNumber(), 'test5');
 
-        $collection = new Collection(8);
+        $collection = Collection::get(8);
         $this->assertEquals($collection->getNumber(), 'test4', 'Test fixture was modified.');
 
         $collection->moveAfterNextSibling();
 
-        $root     = new Collection(1);
+        $root     = Collection::get(1);
         $children = $root->getChildren();
 
         $this->assertEquals(7, count($children));
@@ -915,8 +970,6 @@ protected function setUpFixtureForMoveTests()
      */
     public function testHasVisiblePublishChildren()
     {
-        $this->object->store();
-
         $this->assertFalse($this->object->hasVisiblePublishChildren());
         $this->assertFalse($this->object->hasChildren());
 
@@ -953,8 +1006,6 @@ public function testHasVisiblePublishChildren()
 
     public function testHasVisiblePublishChildrenFalseIfNotVisible()
     {
-        $this->object->store();
-
         $this->assertFalse($this->object->hasVisiblePublishChildren());
         $this->assertFalse($this->object->hasChildren());
 
@@ -974,8 +1025,6 @@ public function testHasVisiblePublishChildrenFalseIfNotVisible()
      */
     public function testGetVisiblePublishChildren()
     {
-        $this->object->store();
-
         // add two children: one of them (the first child) is invisible
         $coll1 = new Collection();
         $coll1->setVisiblePublish('1');
@@ -1019,19 +1068,19 @@ public function testMoveToPositionUp()
     {
         $this->setUpFixtureForMoveTests();
 
-        $root     = new Collection(1);
+        $root     = Collection::get(1);
         $children = $root->getChildren();
         $this->assertEquals(7, count($children));
 
         $this->assertEquals('test3', $children[2]->getNumber(), 'Test fixture was modified.');
         $this->assertEquals('test4', $children[3]->getNumber(), 'Test fixture was modified.');
 
-        $collection = new Collection(8);
+        $collection = Collection::get(8);
         $this->assertEquals('test4', $collection->getNumber(), 'Test fixture was modified.');
 
         $collection->moveToPosition(1);
 
-        $root     = new Collection(1);
+        $root     = Collection::get(1);
         $children = $root->getChildren();
         $this->assertEquals(7, count($children));
 
@@ -1058,13 +1107,13 @@ public function testMoveToPositionDown()
     {
         $this->setUpFixtureForMoveTests();
 
-        $collection = new Collection(4);
+        $collection = Collection::get(4);
 
         $this->assertEquals('test3', $collection->getNumber());
 
         $collection->moveToPosition(5);
 
-        $root     = new Collection(1);
+        $root     = Collection::get(1);
         $children = $root->getChildren();
 
         $this->assertEquals(7, count($children));
@@ -1081,13 +1130,13 @@ public function testMoveToStart()
     {
         $this->setUpFixtureForMoveTests();
 
-        $collection = new Collection(11);
+        $collection = Collection::get(11);
 
         $this->assertEquals('test5', $collection->getNumber());
 
         $collection->moveToStart();
 
-        $root = new Collection(1);
+        $root = Collection::get(1);
 
         $children = $root->getChildren();
 
@@ -1103,13 +1152,13 @@ public function testMoveToEnd()
     {
         $this->setUpFixtureForMoveTests();
 
-        $collection = new Collection(8);
+        $collection = Collection::get(8);
 
         $this->assertEquals('test4', $collection->getNumber());
 
         $collection->moveToEnd();
 
-        $root     = new Collection(1);
+        $root     = Collection::get(1);
         $children = $root->getChildren();
 
         $this->assertEquals(7, count($children));
@@ -1125,11 +1174,11 @@ public function testSortChildrenByName()
     {
         $this->setUpFixtureForMoveTests();
 
-        $root = new Collection(1);
+        $root = Collection::get(1);
 
         $root->sortChildrenByName(true);
 
-        $root     = new Collection(1);
+        $root     = Collection::get(1);
         $children = $root->getChildren();
 
         $this->assertEquals(7, count($children));
@@ -1142,7 +1191,7 @@ public function testSortChildrenByName()
 
         $root->sortChildrenByName();
 
-        $root     = new Collection(1);
+        $root     = Collection::get(1);
         $children = $root->getChildren();
 
         $this->assertEquals(7, count($children));
@@ -1158,11 +1207,11 @@ public function testSortChildrenByNumber()
     {
         $this->setUpFixtureForMoveTests();
 
-        $root = new Collection(1);
+        $root = Collection::get(1);
 
         $root->sortChildrenByName(true);
 
-        $root     = new Collection(1);
+        $root     = Collection::get(1);
         $children = $root->getChildren();
 
         $this->assertEquals(7, count($children));
@@ -1175,7 +1224,7 @@ public function testSortChildrenByNumber()
 
         $root->sortChildrenByName();
 
-        $root     = new Collection(1);
+        $root     = Collection::get(1);
         $children = $root->getChildren();
 
         $this->assertEquals(7, count($children));
@@ -1191,14 +1240,14 @@ public function testSortChildrenBySpecifiedOrder()
     {
         $this->setUpFixtureForMoveTests();
 
-        $root     = new Collection(1);
+        $root     = Collection::get(1);
         $children = $root->getChildren();
 
         $this->assertCount(7, $children);
 
         $root->applySortOrderOfChildren([4, 11, 3, 14, 2, 8, 13]);
 
-        $root     = new Collection(1);
+        $root     = Collection::get(1);
         $children = $root->getChildren();
 
         $this->assertCount(7, $children);
@@ -1218,12 +1267,13 @@ public function testSortChildrenBySpecifiedOrderBadId()
     {
         $this->setUpFixtureForMoveTests();
 
-        $root     = new Collection(1);
+        $root     = Collection::get(1);
         $children = $root->getChildren();
 
         $this->assertCount(7, $children);
 
-        $this->setExpectedException(InvalidArgumentException::class, 'is no child of');
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('is no child of');
 
         $root->applySortOrderOfChildren([4, 11, 3, 16, 2, 8, 13]);
     }
@@ -1233,18 +1283,14 @@ public function testSortChildrenBySpecifiedOrderBadId()
      */
     protected function validateNestedSet()
     {
-        $table = new Collections();
-
-        $select = $table->select()->where('role_id = ?', 1)->order('left_id ASC');
-
-        $rows = $table->fetchAll($select);
+        $rows = Collection::fetchCollectionsByRoleId(1, true);
 
         $this->assertEquals(14, count($rows));
 
-        $this->assertEquals(1, $rows[0]->left_id);
-        $this->assertEquals(28, $rows[0]->right_id);
+        $this->assertEquals(1, $rows[0]->getLeft());
+        $this->assertEquals(28, $rows[0]->getRight());
 
-        $validator = new NestedSetValidator($table);
+        $validator = new NestedSetValidator();
 
         $this->assertTrue($validator->validate(1));
     }
@@ -1288,7 +1334,7 @@ public function testHandlingOfNullValues()
 
         $collection->store();
 
-        $collection = new Collection($collection->getId());
+        $collection = Collection::get($collection->getId());
 
         $this->assertNull($collection->getNumber());
         $this->assertNull($collection->getOaiSubset());
diff --git a/tests/Opus/TestAsset/AbstractSimpleTestCase.php b/tests/Opus/TestAsset/AbstractSimpleTestCase.php
index 7efc365c..53e5c483 100644
--- a/tests/Opus/TestAsset/AbstractSimpleTestCase.php
+++ b/tests/Opus/TestAsset/AbstractSimpleTestCase.php
@@ -35,7 +35,6 @@
 namespace OpusTest\TestAsset;
 
 use Opus\Config;
-use PHPUnit_Framework_TestCase;
 use Zend_Config;
 
 use function call_user_func;
@@ -48,7 +47,7 @@
  *
  * TODO needs refactoring
  */
-abstract class AbstractSimpleTestCase extends PHPUnit_Framework_TestCase
+abstract class AbstractSimpleTestCase extends \PHPUnit\Framework\TestCase
 {
     private $configBackup;
 
diff --git a/tests/Opus/TestAsset/NestedSetValidator.php b/tests/Opus/TestAsset/NestedSetValidator.php
index 9221f0cb..6d9532f3 100644
--- a/tests/Opus/TestAsset/NestedSetValidator.php
+++ b/tests/Opus/TestAsset/NestedSetValidator.php
@@ -35,9 +35,8 @@
 
 namespace OpusTest\TestAsset;
 
-use Opus\Db\NestedSet;
 use Opus\LoggingTrait;
-use Opus\Model\ModelException;
+use Opus\Model2\Collection;
 
 /**
  * Validates the structure of a NestedSet in the database.
@@ -46,13 +45,6 @@ class NestedSetValidator
 {
     use LoggingTrait;
 
-    /**
-     * Database table adapter.
-     *
-     * @var NestedSet
-     */
-    private $table;
-
     /**
      * Position counter in NestedSet.
      *
@@ -60,19 +52,6 @@ class NestedSetValidator
      */
     private $counter;
 
-    /**
-     * @param NestedSet $table
-     * @throws ModelException
-     */
-    public function __construct($table)
-    {
-        if ($table !== null && $table instanceof NestedSet) {
-            $this->table = $table;
-        } else {
-            throw new ModelException('object must be instance of Opus\Db\NestedSet');
-        }
-    }
-
     /**
      * Check structure of nested set.
      *
@@ -81,9 +60,9 @@ public function __construct($table)
      */
     public function validate($rootId)
     {
-        $select        = $this->table->select()->where('id = ?', $rootId);
-        $node          = $this->table->fetchRow($select);
-        $this->counter = (int) $node['left_id'];
+        /** @var Collection $node */
+        $node          = Collection::get($rootId);
+        $this->counter = $node->getLeft();
         return $this->validateNode($rootId); // root node
     }
 
@@ -97,10 +76,10 @@ public function validateNode($nodeId)
     {
         $logger = $this->getLogger();
 
-        $select  = $this->table->select()->where('id = ?', $nodeId);
-        $node    = $this->table->fetchRow($select);
-        $leftId  = $node['left_id'];
-        $rightId = $node['right_id'];
+        /** @var Collection $node */
+        $node    = Collection::get($nodeId);
+        $leftId  = $node->getLeft();
+        $rightId = $node->getRight();
 
         $logger->err("{$this->counter}, $nodeId: $leftId, $rightId");
 
@@ -121,13 +100,9 @@ public function validateNode($nodeId)
             // node
             if ($distance & 1) {
                 // odd; valid
-                $selectChildren = $this->table->select()->where(
-                    'parent_id  = ?',
-                    $nodeId
-                )->order('left_id ASC');
-                 $children      = $this->table->fetchAll($selectChildren);
+                $children = Collection::fetchChildrenByParentId($nodeId, true);
                 foreach ($children as $child) {
-                    if ($this->validateNode($child['id']) === false) {
+                    if ($this->validateNode($child->getId()) === false) {
                         return false;
                     }
                 }