Add support for relationship-level links and meta.

test/adapter/json_api/links_test.rb

@@ -17,11 +17,23 @@ 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
             @post = Post.new(id: 1337, comments: [], author: nil)
-            @author = LinkAuthor.new(id: 1337)
+            @author = LinkAuthor.new(id: 1337, posts: [@post])
           end
 
           def test_toplevel_links
@@ -61,6 +73,23 @@ 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