Add support for resource-level JSON API links.

test/adapter/json_api/links_test.rb

@@ -5,8 +5,19 @@ module ActiveModel
     module Adapter
       class JsonApi
         class LinksTest < Minitest::Test
+          LinkAuthor = Class.new(::Model)
+          class LinkAuthorSerializer < ActiveModel::Serializer
+            link :self do
+              href "//example.com/link_author/#{object.id}"
+              meta stuff: 'value'
+            end
+
+            link :other, '//example.com/resource'
+          end
+
           def setup
             @post = Post.new(id: 1337, comments: [], author: nil)
+            @author = LinkAuthor.new(id: 1337)
           end
 
           def test_toplevel_links
@@ -15,16 +26,36 @@ module ActiveModel
               adapter: :json_api,
               links: {
                 self: {
-                  href: '//posts'
+                  href: '//example.com/posts',
+                  meta: {
+                    stuff: 'value'
+                  }
                 }
               }).serializable_hash
             expected = {
               self: {
-                href: '//posts'
+                href: '//example.com/posts',
+                meta: {
+                  stuff: 'value'
+                }
               }
             }
             assert_equal(expected, hash[:links])
           end
+
+          def test_resource_links
+            hash = serializable(@author, adapter: :json_api).serializable_hash
+            expected = {
+              self: {
+                href: '//example.com/link_author/1337',
+                meta: {
+                  stuff: 'value'
+                }
+              },
+              other: '//example.com/resource'
+            }
+            assert_equal(expected, hash[:data][:links])
+          end
         end
       end
     end