Add key option to serializer associations

test/adapter/json_test.rb

@@ -7,7 +7,7 @@ module ActiveModel
         def setup
           ActionController::Base.cache_store.clear
           @author = Author.new(id: 1, name: 'Steve K.')
-          @post = Post.new(title: 'New Post', body: 'Body')
+          @post = Post.new(id: 1, title: 'New Post', body: 'Body')
           @first_comment = Comment.new(id: 1, body: 'ZOMG A COMMENT')
           @second_comment = Comment.new(id: 2, body: 'ZOMG ANOTHER COMMENT')
           @post.comments = [@first_comment, @second_comment]
@@ -27,6 +27,20 @@ module ActiveModel
                          {id: 2, body: 'ZOMG ANOTHER COMMENT'}
                        ], @adapter.serializable_hash[:post][:comments])
         end
+
+        def test_custom_keys
+          serializer = PostWithCustomKeysSerializer.new(@post)
+          adapter = ActiveModel::Serializer::Adapter::Json.new(serializer)
+
+          assert_equal({
+            id: 1,
+            reviews: [{id: 1, body: "ZOMG A COMMENT"},
+                      {id: 2, body: "ZOMG ANOTHER COMMENT"}
+                    ],
+            writer: {id: 1, name: "Steve K."},
+            site: {id: 1, name: "My Blog!!"}
+            }, adapter.serializable_hash[:post_with_custom_keys])
+        end
       end
     end
   end