Delegate missing Rails::Initializable::Collection methods to inner array

Previously, Rails::Initializable::Collection extended Array, but now it
doesn't. The inner structure has changed enough so that we need to
implement addition methods, but we can keep every other subtractive
method as-is.

railties/lib/rails/initializable.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "tsort"
+require "active_support/core_ext/module/delegation"
 
 module Rails
   module Initializable
@@ -38,6 +39,8 @@ class Collection
       include Enumerable
       include TSort
 
+      delegate_missing_to :@collection
+
       def initialize(initializers = nil)
         @order = Hash.new { |hash, key| hash[key] = Set.new }
         @resolve = Hash.new { |hash, key| hash[key] = Set.new }
@@ -73,13 +76,23 @@ def <<(initializer)
         @order[initializer.before] << initializer.name if initializer.before
         @order[initializer.name] << initializer.after if initializer.after
         @resolve[initializer.name] << initializer
+        self
       end
 
-      def concat(initializers)
+      def push(*initializers)
         initializers.each(&method(:<<))
         self
       end
 
+      alias_method(:append, :push)
+
+      def concat(*initializer_collections)
+        initializer_collections.each do |initializers|
+          initializers.each(&method(:<<))
+        end
+        self
+      end
+
       def has?(name)
         @resolve.key?(name)
       end

railties/test/initializable_test.rb

@@ -277,4 +277,64 @@ class OverriddenInitializerTest < ActiveSupport::TestCase
       assert_equal [1, 2, 3, 4], $arr
     end
   end
+
+  class CollectionTest < ActiveSupport::TestCase
+    test "delegates missing to collection array" do
+      initializable = Class.new do
+        include Rails::Initializable
+      end
+
+      Array.public_instance_methods.each do |method_name|
+        assert(
+          initializable.initializers.respond_to?(method_name),
+          "Expected Initializable::Collection to respond to #{method_name}, but does not.",
+        )
+      end
+    end
+
+    test "concat" do
+      one = collection(:a, :b)
+      two = collection(:c, :d)
+      initializers = one.initializers.concat(two.initializers)
+      initializer_names = initializers.tsort_each.map(&:name)
+
+      assert_equal [:a, :b, :c, :d], initializer_names
+    end
+
+    test "push" do
+      one = collection(:a, :b, :c)
+      two = collection(:d)
+      initializers = one.initializers.push(two.initializers.first)
+      initializer_names = initializers.tsort_each.map(&:name)
+
+      assert_equal [:a, :b, :c, :d], initializer_names
+    end
+
+    test "append" do
+      one = collection(:a)
+      two = collection(:b, :c)
+      initializers = one.initializers.append(two.initializers.first)
+      initializer_names = initializers.tsort_each.map(&:name)
+
+      assert_equal [:a, :b], initializer_names
+    end
+
+    test "<<" do
+      one = collection(:a, :b)
+      two = collection(:c)
+      initializers = (one.initializers << two.initializers.first)
+      initializer_names = initializers.tsort_each.map(&:name)
+
+      assert_equal [:a, :b, :c], initializer_names
+    end
+
+    private
+
+    def collection(*names)
+      Class.new do
+        include Rails::Initializable
+        names.each { |name| initializer(name) { } }
+      end
+    end
+  end
 end