Support passing a custom ArraySerializer for has_many associations

Thanks @phoet

Closes #316

lib/active_model/array_serializer.rb

@@ -30,6 +30,6 @@ module ActiveModel
         serializer.new(item, @options).serializable_object
       end
     end
-    alias serializable_object serializable_array
+    alias_method :serializable_object, :serializable_array
   end
 end

lib/active_model/serializer.rb

@@ -158,10 +158,12 @@ end
 
     def serialize(association)
       associated_data = send(association.name)
-      if associated_data.respond_to?(:to_ary)
+      if associated_data.respond_to?(:to_ary) &&
+         !(association.serializer_class &&
+           association.serializer_class <= ArraySerializer)
         associated_data.map { |elem| association.build_serializer(elem).serializable_hash }
       else
-        association.build_serializer(associated_data).serializable_hash
+        association.build_serializer(associated_data).serializable_object
       end
     end
 
@@ -179,6 +181,6 @@ end
       hash = attributes
       hash.merge! associations
     end
-    alias serializable_object serializable_hash
+    alias_method :serializable_object, :serializable_hash
   end
 end

test/unit/active_model/serializer/has_many_test.rb

@@ -129,6 +129,21 @@ module ActiveModel
           comments: [{ content: 'fake' }, { content: 'fake' }]
         }, @post_serializer.as_json)
       end
+
+      def test_associations_using_a_given_array_serializer
+        @association.embed = :ids
+        @association.embed_in_root = true
+        @association.serializer_class = Class.new(ActiveModel::ArraySerializer) do
+          def serializable_object
+            { my_content: ['fake'] }
+          end
+        end
+
+        assert_equal({
+          'post' => { title: 'Title 1', body: 'Body 1', 'comment_ids' => @post.comments.map { |c| c.object_id } },
+          comments: { my_content: ['fake'] }
+        }, @post_serializer.as_json)
+      end
     end
   end
 end