Associations refactoring

* Move all associations related code from Serializer class to Associations module
* Introduce Reflection class hierarchy
* Introduce Association class
* Rid off Serializer#each_association
* Introduce Serializer#associations enumerator

lib/active_model/serializer/associations.rb

@@ -0,0 +1,107 @@
+module ActiveModel
+  class Serializer
+    # Defines an association in the object should be rendered.
+    #
+    # The serializer object should implement the association name
+    # as a method which should return an array when invoked. If a method
+    # with the association name does not exist, the association name is
+    # dispatched to the serialized object.
+    #
+    module Associations
+      extend ActiveSupport::Concern
+
+      included do |base|
+        class << base
+          attr_accessor :_reflections
+        end
+
+        autoload :Association
+        autoload :Reflection
+        autoload :SingularReflection
+        autoload :CollectionReflection
+        autoload :BelongsToReflection
+        autoload :HasOneReflection
+        autoload :HasManyReflection
+      end
+
+      module ClassMethods
+        def inherited(base)
+          base._reflections = self._reflections.try(:dup) || []
+        end
+
+        # @param [Array(Array<Symbol>, Hash{Symbol => Object})] attrs
+        # @return [void]
+        #
+        # @example
+        #  has_many :comments, serializer: CommentSummarySerializer
+        #  has_many :commits, authors
+        #
+        def has_many(*attrs)
+          associate attrs do |name, options|
+            HasManyReflection.new(name, options)
+          end
+        end
+
+        # @param [Array(Array<Symbol>, Hash{Symbol => Object})] attrs
+        # @return [void]
+        #
+        # @example
+        #  belongs_to :author, serializer: AuthorSerializer
+        #
+        def belongs_to(*attrs)
+          associate attrs do |name, options|
+            BelongsToReflection.new(name, options)
+          end
+        end
+
+        # @param [Array(Array<Symbol>, Hash{Symbol => Object})] attrs
+        # @return [void]
+        #
+        # @example
+        #  has_one :author, serializer: AuthorSerializer
+        #
+        def has_one(*attrs)
+          associate attrs do |name, options|
+            HasOneReflection.new(name, options)
+          end
+        end
+
+        private
+
+        # Add reflection and define {name} accessor.
+        # @param [Array<Symbol>]
+        # @yield [Symbol] return reflection
+        #
+        # @api private
+        #
+        def associate(attrs)
+          options = attrs.extract_options!
+
+          self._reflections = _reflections.dup
+
+          attrs.each do |name|
+            unless method_defined?(name)
+              define_method name do
+                object.send name
+              end
+            end
+
+            self._reflections << yield(name, options)
+          end
+        end
+      end
+
+      # @return [Enumerator<Association>]
+      #
+      def associations
+        return unless object
+
+        Enumerator.new do |y|
+          self.class._reflections.each do |reflection|
+            y.yield reflection.build_association(self, options)
+          end
+        end
+      end
+    end
+  end
+end