DOCSP-42606: class maps feedback (#592)

* DOCSP-42606: class maps improvements

* wip

* RM PR fixes 1

* BD tech review comments 1

* BD tech review comments 2

Author: rustagir

source/fundamentals/serialization/class-mapping.txt

@@ -21,10 +21,13 @@ Overview
 --------
 
 In this guide, you can learn how to customize the way the {+driver-long+}
-maps BSON documents to and from {+language+} classes. You should read this page
+maps BSON documents to and from {+language+} classes. You can read this page
 to learn more about the default class mapping behavior, or if you need to
 customize the way the driver serializes or deserializes your data.
 
+You can also use POCO attributes to set serialization behavior. To learn
+more, see the :ref:`csharp-poco` guide.
+
 Automatic Class Mapping
 -----------------------
 
@@ -69,7 +72,7 @@ class:
        classMap.MapMember(p => p.Hobbies);
    });
 
-.. important::
+.. important:: When to Register a Class Map
 
    You must register a class map *before* it's needed in your code. We recommend
    registering class maps prior to initializing a connection with MongoDB.
@@ -95,9 +98,34 @@ Customize Class Serialization
 -----------------------------
 
 You can customize how the driver serializes and deserializes documents at the class
-level by using attributes with the class or by calling methods while registering
+level by applying attributes to the class or by calling methods while registering
 a class map.
 
+.. _csharp-class-map-omit-fields:
+
+Omit Fields
+~~~~~~~~~~~
+
+You can prevent specified fields from being serialized by using the
+``UnmapMember()`` method when registering a class map.
+
+The following example shows how to create a class map to prevent the
+``Age`` property from being serialized:
+
+.. code-block:: csharp
+   :emphasize-lines: 4
+
+   BsonClassMap.RegisterClassMap<Person>(classMap => 
+   {
+        classMap.AutoMap();
+        classMap.UnmapMember(p => p.Age);
+   });
+
+.. tip::
+
+   To learn about how to set this behavior by using a field attribute,
+   see the :ref:`csharp-poco-omit-fields` section of the POCOs guide.
+
 Ignore Extra Elements
 ~~~~~~~~~~~~~~~~~~~~~
 
@@ -134,6 +162,89 @@ You can also ignore any extra elements when registering a class map:
         classMap.SetIgnoreExtraElements(true);
    });
 
+Identify Id Field
+~~~~~~~~~~~~~~~~~
+
+By default, the driver maps any public property named ``Id``, ``id``, or
+``_id`` to the BSON ``_id`` field. To explicitly select the
+property to map to the ``_id`` field, use the ``MapIdMember()`` class
+map method.
+
+The following code sample maps the ``Identifier`` property to the
+``_id`` field:
+
+.. code-block:: csharp
+   :emphasize-lines: 4
+   
+   BsonClassMap.RegisterClassMap<Person>(classMap => 
+   {
+        classMap.AutoMap();
+        classMap.MapIdMember(p => p.Identifier);
+   });
+
+String IDs
+``````````
+
+If you are using strings to represent your
+documement IDs, you can use the ``IdMemberMap.SetSerializer()`` class
+map method to serialize these values to ``ObjectId`` instances in
+MongoDB. This way, you can customize how your ID values are represented
+in your application while adhering to MongoDB best practices for ID
+management in the database.
+
+The following example directs the driver to serialize string ID values
+as ``ObjectId`` values:
+
+.. code-block:: csharp
+   :emphasize-lines: 4
+
+   BsonClassMap.RegisterClassMap<Person>(classMap => 
+   {
+        classMap.AutoMap();
+        classMap.IdMemberMap.SetSerializer(new StringSerializer(BsonType.ObjectId));
+   });
+
+If you want the driver to generate valid 24-digit hex strings to
+use as your ID values, you can chain the ``SetIdGenerator()`` method to
+the ``MapIdMember()`` method and pass an instance of
+``StringObjectIdGenerator``:
+
+.. code-block:: csharp
+   :emphasize-lines: 4
+
+   BsonClassMap.RegisterClassMap<Person>(classMap => 
+   {
+        classMap.AutoMap();
+        classMap.MapIdMember(p => p.Identifier).SetIdGenerator(StringObjectIdGenerator.Instance);
+   });
+
+To learn more about ID generators, see the
+:ref:`csharp-poco-specify-id-gen` section of the POCOs guide.
+
+GUID IDs
+````````
+
+If your application uses GUIDs as unique identifiers in your documents,
+you can register a class map to specify how ``Guid`` values are
+serialized. By default, the driver uses the
+``GuidGenerator`` type to generate a unique value for the ID property.
+You must specify the GUID representation by initializing a
+``GuidSerializer``.
+
+The following code specifies the ``Standard`` GUID representation when
+registering the class map:
+
+.. code-block:: csharp
+   :emphasize-lines: 4
+
+   BsonClassMap.RegisterClassMap<Person>(classMap => 
+   {
+        classMap.AutoMap();
+        classMap.IdMemberMap.SetSerializer(new GuidSerializer(GuidRepresentation.Standard));
+   });
+
+To learn more about serializing GUIDs, see the :ref:`csharp-guids` guide.
+
 Mapping with Constructors
 -------------------------
 

source/fundamentals/serialization/poco.txt

@@ -266,6 +266,8 @@ The following code sample maps the ``Identifier`` property to the
    The ``_id`` field mapping logic described in this section only applies to the
    root document and does not apply to nested documents.
 
+.. _csharp-poco-specify-id-gen:
+
 Specify an ID Generator
 ```````````````````````
 
@@ -312,7 +314,9 @@ The following table describes the ID generators available in the
           public class House
           {
              public Guid Id { get; set; }
-          } 
+          }
+
+       To learn more, see the :ref:`csharp-guids` guide.
 
    * - ``ObjectId``
      - The driver automatically uses the ``ObjectIdGenerator`` type for ID properties with
@@ -427,13 +431,45 @@ the default value (``null``), the driver throws an exception:
        public Guid Id { get; set; }
    }
 
+.. _csharp-poco-omit-fields:
+
+Omit Fields
+~~~~~~~~~~~
+
+To prevent the driver from serializing a specified field, use the
+``[BsonIgnore]`` attribute. The following code shows how you can prevent
+the driver from serializing the ``YearBuilt`` property:
+
+.. code-block:: csharp
+   :copyable: true
+   :emphasize-lines: 5
+
+   public class House
+   {
+       public Guid Id { get; set; }
+
+       [BsonIgnore]
+       public int YearBuilt { get; set; }
+       public string Style { get; set; }
+   }
+
+Even if you define the ``YearBuilt`` property, the field is
+not saved in MongoDB.
+
+.. note::
+
+   You can define a class map to prevent specific fields from being
+   serialized. To learn more and view an example, see the
+   :ref:`csharp-class-map-omit-fields` section of the Class Mapping
+   guide.
+
 Omit Empty Fields
 ~~~~~~~~~~~~~~~~~
 
-By default, the driver serializes undefined properties to fields with ``null``
-values. To ignore undefined properties during serialization, use the ``[BsonIgnore]``
+By default, the driver serializes uninitialized properties with ``null``
+values. To ignore empty properties during serialization, use the ``[BsonIgnoreIfNull]``
 attribute. The following code shows how you can prevent the driver from
-serializing the ``YearBuilt`` property if it is undefined:
+serializing the ``Style`` property if it is uninitialized:
 
 .. code-block:: csharp
    :copyable: true
@@ -443,11 +479,36 @@ serializing the ``YearBuilt`` property if it is undefined:
    {
        public Guid Id { get; set; }
 
-       [BsonIgnore]
-       public int YearBuilt { get; set; }
+       [BsonIgnoreIfNull]
        public string Style { get; set; }
+       public int YearBuilt { get; set; }
    }
 
+You can also instruct the driver to ignore a property that contains a
+``null`` value when you register the class map, as shown in the
+following example:
+
+.. code-block:: csharp
+   :copyable: true
+   :emphasize-lines: 4
+
+   BsonClassMap.RegisterClassMap<House>(classMap => 
+   {
+      classMap.AutoMap();
+      classMap.MapMember(h => h.Style).SetIgnoreIfNull(true);
+   });
+
+.. note:: Value Type Properties
+
+   You cannot use the ``[BsonIgnoreIfNull]`` attribute or
+   ``SetIgnoreIfNull()`` method to prevent uninitialized value type
+   properties from being serialized unless you mark the properties as
+   nullable. Instead, use the ``[BsonIgnoreIfDefault]`` attribute or
+   ``SetIgnoreIfDefault()`` class map method, which are described in the
+   :ref:`csharp-poco-default-values` section of this guide.
+
+.. _csharp-poco-default-values:
+
 Customize Default Values
 ~~~~~~~~~~~~~~~~~~~~~~~~