Fix relationship behavior when using block

When using the relationships DSL with a block e.g.
has_one :bio do
  link :self, "some_link"
end
the "data" field would be rendered with a nil value even though the bio
is not nil. This happened because the block return value was set to nil
but used as a value for the "data" field.

lib/active_model/serializer/reflection.rb

@@ -42,17 +42,17 @@ module ActiveModel
 
       def link(name, value = nil, &block)
         @_links[name] = block || value
-        nil
+        :nil
       end
 
       def meta(value = nil, &block)
         @_meta = block || value
-        nil
+        :nil
       end
 
       def include_data(value = true)
         @_include_data = value
-        nil
+        :nil
       end
 
       def value(serializer)
@@ -60,7 +60,12 @@ module ActiveModel
         @scope = serializer.scope
 
         if block
-          instance_eval(&block)
+          block_value = instance_eval(&block)
+          if block_value == :nil
+            serializer.read_attribute_for_serialization(name)
+          else
+            block_value
+          end
         else
           serializer.read_attribute_for_serialization(name)
         end

test/adapter/json_api/links_test.rb

@@ -17,18 +17,6 @@ module ActiveModel
             link :yet_another do
               "//example.com/resource/#{object.id}"
             end
-
-            has_many :posts do
-              link :self do
-                href '//example.com/link_author/relationships/posts'
-                meta stuff: 'value'
-              end
-              link :related do
-                href '//example.com/link_author/posts'
-                meta count: object.posts.count
-              end
-              include_data false
-            end
           end
 
           def setup
@@ -91,23 +79,6 @@ module ActiveModel
             }
             assert_equal(expected, hash[:data][:links])
           end
-
-          def test_relationship_links
-            hash = serializable(@author, adapter: :json_api).serializable_hash
-            expected = {
-              links: {
-                self: {
-                  href: '//example.com/link_author/relationships/posts',
-                  meta: { stuff: 'value' }
-                },
-                related: {
-                  href: '//example.com/link_author/posts',
-                  meta: { count: 1 }
-                }
-              }
-            }
-            assert_equal(expected, hash[:data][:relationships][:posts])
-          end
         end
       end
     end

test/adapter/json_api/relationship_test.rb

@@ -0,0 +1,173 @@
+require 'test_helper'
+
+module ActiveModel
+  class Serializer
+    module Adapter
+      class JsonApi
+        class RelationshipTest < ActiveSupport::TestCase
+          RelationshipAuthor = Class.new(::Model)
+          class RelationshipAuthorSerializer < ActiveModel::Serializer
+            has_one :bio do
+              link :self, '//example.com/link_author/relationships/bio'
+            end
+
+            has_one :profile do
+              link :related do
+                "//example.com/profiles/#{object.profile.id}"
+              end
+            end
+
+            has_many :locations do
+              link :related do
+                ids = object.locations.map!(&:id).join(',')
+                href "//example.com/locations/#{ids}"
+              end
+            end
+
+            has_many :posts do
+              link :related do
+                ids = object.posts.map!(&:id).join(',')
+                href "//example.com/posts/#{ids}"
+                meta ids: ids
+              end
+            end
+
+            has_many :roles do
+              meta count: object.posts.count
+            end
+
+            has_one :blog do
+              link :self, '//example.com/link_author/relationships/blog'
+              include_data false
+            end
+
+            belongs_to :reviewer do
+              meta name: 'Dan Brown'
+              include_data true
+            end
+
+            has_many :likes do
+              link :related do
+                ids = object.likes.map!(&:id).join(',')
+                href "//example.com/likes/#{ids}"
+                meta ids: ids
+              end
+              meta liked: object.likes.any?
+            end
+          end
+
+          def setup
+            @post = Post.new(id: 1337, comments: [], author: nil)
+            @blog = Blog.new(id: 1337, name: 'extra')
+            @bio = Bio.new(id: 1337)
+            @like = Like.new(id: 1337)
+            @role = Role.new(id: 1337)
+            @profile = Profile.new(id: 1337)
+            @location = Location.new(id: 1337)
+            @reviewer = Author.new(id: 1337)
+            @author = RelationshipAuthor.new(
+              id: 1337,
+              posts: [@post],
+              blog: @blog,
+              reviewer: @reviewer,
+              bio: @bio,
+              likes: [@like],
+              roles: [@role],
+              locations: [@location],
+              profile: @profile
+            )
+          end
+
+          def test_relationship_simple_link
+            hash = serializable(@author, adapter: :json_api).serializable_hash
+            expected = {
+              data: {
+                id: '1337',
+                type: 'bios'
+              },
+              links: {
+                self: '//example.com/link_author/relationships/bio'
+              }
+            }
+            assert_equal(expected, hash[:data][:relationships][:bio])
+          end
+
+          def test_relationship_block_link
+            hash = serializable(@author, adapter: :json_api).serializable_hash
+            expected = {
+              data: { id: '1337', type: 'profiles' },
+              links: { related: '//example.com/profiles/1337' }
+            }
+            assert_equal(expected, hash[:data][:relationships][:profile])
+          end
+
+          def test_relationship_block_link_href
+            hash = serializable(@author, adapter: :json_api).serializable_hash
+            expected = {
+              data: [{ id: '1337', type: 'locations' }],
+              links: {
+                related: { href: '//example.com/locations/1337' }
+              }
+            }
+            assert_equal(expected, hash[:data][:relationships][:locations])
+          end
+
+          def test_relationship_block_link_meta
+            hash = serializable(@author, adapter: :json_api).serializable_hash
+            expected = {
+              data: [{ id: '1337', type: 'posts' }],
+              links: {
+                related: {
+                  href: '//example.com/posts/1337',
+                  meta: { ids: '1337' }
+                }
+              }
+            }
+            assert_equal(expected, hash[:data][:relationships][:posts])
+          end
+
+          def test_relationship_meta
+            hash = serializable(@author, adapter: :json_api).serializable_hash
+            expected = {
+              data: [{ id: '1337', type: 'roles' }],
+              meta: { count: 1 }
+            }
+            assert_equal(expected, hash[:data][:relationships][:roles])
+          end
+
+          def test_relationship_not_including_data
+            hash = serializable(@author, adapter: :json_api).serializable_hash
+            expected = {
+              links: { self: '//example.com/link_author/relationships/blog' }
+            }
+            assert_equal(expected, hash[:data][:relationships][:blog])
+          end
+
+          def test_relationship_including_data_explicit
+            hash = serializable(@author, adapter: :json_api).serializable_hash
+            expected = {
+              data: { id: '1337', type: 'authors' },
+              meta: { name: 'Dan Brown' }
+            }
+            assert_equal(expected, hash[:data][:relationships][:reviewer])
+          end
+
+          def test_relationship_with_everything
+            hash = serializable(@author, adapter: :json_api).serializable_hash
+            expected = {
+              data: [{ id: '1337', type: 'likes' }],
+              links: {
+                related: {
+                  href: '//example.com/likes/1337',
+                  meta: { ids: '1337' }
+                }
+              },
+              meta: { liked: true }
+            }
+            assert_equal(expected, hash[:data][:relationships][:likes])
+          end
+        end
+      end
+    end
+  end
+end