Encapsulate serialized_associations; test inline associations

lib/active_model/serializer/associations.rb

@@ -13,7 +13,9 @@ module Associations
       DEFAULT_INCLUDE_TREE = ActiveModel::Serializer::IncludeTree.from_string('*')
 
       included do |base|
-        base.class_attribute :_reflections
+        base.class_attribute :serialized_associations, instance_writer: false # @api public: maps association name to 'Reflection' instance
+        base.serialized_associations ||= {}
+        base.class_attribute :_reflections, instance_writer: false
         base._reflections ||= []
 
         extend ActiveSupport::Autoload
@@ -77,13 +79,16 @@ def has_one(name, options = {}, &block)
         def associate(reflection, block)
           self._reflections = _reflections.dup
 
-          define_method reflection.name do
-            if block_given?
-              instance_eval(&block)
-            else
-              object.send reflection.name
-            end
-          end unless method_defined?(reflection.name)
+          reflection_name = reflection.name
+          if block
+            serialized_associations[reflection_name] = ->(instance) { instance.instance_eval(&block) }
+          else
+            serialized_associations[reflection_name] = ->(instance) { instance.object.send(reflection_name) }
+          end
+
+          define_method reflection_name do
+            serialized_associations[reflection_name].call(self)
+          end unless method_defined?(reflection_name)
 
           self._reflections << reflection
         end

test/serializers/associations_test.rb

@@ -126,6 +126,35 @@ def test_associations_custom_keys
         assert expected_association_keys.include? :site
       end
 
+      class InlineAssociationTestPostSerializer < ActiveModel::Serializer
+        has_many :comments
+        has_many :last_comments do
+          object.comments.last(1)
+        end
+      end
+
+      def test_virtual_attribute_block
+        comment1 = ::ARModels::Comment.create!(contents: 'first comment')
+        comment2 = ::ARModels::Comment.create!(contents: 'last comment')
+        post = ::ARModels::Post.create!(
+          title: 'inline association test',
+          body: 'etc',
+          comments: [comment1, comment2]
+        )
+        actual = serializable(post, adapter: :attributes, serializer: InlineAssociationTestPostSerializer).as_json
+        expected = {
+          :comments =>           [
+            { :id => 1, :contents => 'first comment' },
+            { :id => 2, :contents => 'last comment' }
+          ],
+          :last_comments =>           [
+            { :id => 2, :contents => 'last comment' }
+          ]
+        }
+
+        assert_equal expected, actual
+      end
+
       class NamespacedResourcesTest < Minitest::Test
         class ResourceNamespace
           Post    = Class.new(::Model)