Evaluate association blocks as scopes on the association

lib/active_model/serializer/reflection.rb

@@ -7,8 +7,16 @@ module ActiveModel
     #  class PostSerializer < ActiveModel::Serializer
     #     has_one :author, serializer: AuthorSerializer
     #     has_many :comments
+    #     has_many :comments, key: :last_comments do
+    #       last(1)
+    #     end
     #  end
     #
+    #  Notice that the association block is evaluated in the context of the association.
+    #  Specifically, the association 'comments' is evaluated two different ways:
+    #  1) as 'comments' and named 'comments'.
+    #  2) as 'comments.last(1)' and named 'last_comments'.
+    #
     #  PostSerializer._reflections #=>
     #    # [
     #    #   HasOneReflection.new(:author, serializer: AuthorSerializer),
@@ -33,7 +41,7 @@ module ActiveModel
 
       def self.build_reader(name, block)
         if block
-          ->(instance) { instance.instance_eval(&block) }
+          ->(instance) { instance.read_attribute_for_serialization(name).instance_eval(&block) }
         else
           ->(instance) { instance.read_attribute_for_serialization(name) }
         end

test/serializers/associations_test.rb

@@ -128,8 +128,8 @@ module ActiveModel
 
       class InlineAssociationTestPostSerializer < ActiveModel::Serializer
         has_many :comments
-        has_many :last_comments do
+        has_many :comments, key: :last_comments do
-          object.comments.last(1)
+          last(1)
         end
       end