Add support for resource-level JSON API links.

lib/active_model/serializer.rb

@@ -50,6 +50,9 @@ module ActiveModel
       self._attributes ||= []
       class_attribute :_attributes_keys          # @api private : maps attribute value to explict key name, @see Serializer#attribute
       self._attributes_keys ||= {}
+      class_attribute :_links                    # @api private : links definitions, @see Serializer#link
+      self._links ||= {}
+
       serializer.class_attribute :_cache         # @api private : the cache object
       serializer.class_attribute :_fragmented    # @api private : @see ::fragmented
       serializer.class_attribute :_cache_key     # @api private : when present, is first item in cache_key
@@ -72,6 +75,7 @@ module ActiveModel
       caller_line = caller.first
       base._attributes = _attributes.dup
       base._attributes_keys = _attributes_keys.dup
+      base._links = _links.dup
       base._cache_digest = digest_caller_file(caller_line)
       super
     end
@@ -83,6 +87,10 @@ module ActiveModel
       self._type = type
     end
 
+    def self.link(name, value = nil, &block)
+      _links[name] = block || value
+    end
+
     # @example
     #   class AdminAuthorSerializer < ActiveModel::Serializer
     #     attributes :id, :name, :recent_edits
@@ -249,6 +257,12 @@ module ActiveModel
       end
     end
 
+    # @api private
+    # Used by JsonApi adapter to build resource links.
+    def links
+      self.class._links
+    end
+
     protected
 
     attr_accessor :instance_options

lib/active_model/serializer/adapter/json_api.rb

@@ -5,6 +5,7 @@ module ActiveModel
         extend ActiveSupport::Autoload
         autoload :PaginationLinks
         autoload :FragmentCache
+        autoload :Link
 
         # TODO: if we like this abstraction and other API objects to it,
         # then extract to its own file and require it.
@@ -94,7 +95,7 @@ module ActiveModel
 
           if serializer.paginated?
             hash[:links] ||= {}
-            hash[:links].update(links_for(serializer, options))
+            hash[:links].update(pagination_links_for(serializer, options))
           end
 
           hash
@@ -136,12 +137,21 @@ module ActiveModel
           { id: id.to_s, type: type }
         end
 
+        def attributes_for(serializer, fields)
+          serializer.attributes(fields).except(:id)
+        end
+
         def resource_object_for(serializer)
           cache_check(serializer) do
             resource_object = resource_identifier_for(serializer)
+
             requested_fields = fieldset && fieldset.fields_for(resource_object[:type])
-            attributes = serializer.attributes(requested_fields).except(:id)
+            attributes = attributes_for(serializer, requested_fields)
             resource_object[:attributes] = attributes if attributes.any?
+
+            links = links_for(serializer)
+            resource_object[:links] = links if links.any?
+
             resource_object
           end
         end
@@ -204,7 +214,21 @@ module ActiveModel
           end
         end
 
-        def links_for(serializer, options)
+        def links_for(serializer)
+          serializer.links.each_with_object({}) do |(name, value), hash|
+            hash[name] =
+              if value.respond_to?(:call)
+                link = Link.new(serializer)
+                link.instance_eval(&value)
+
+                link.to_hash
+              else
+                value
+              end
+          end
+        end
+
+        def pagination_links_for(serializer, options)
           JsonApi::PaginationLinks.new(serializer.object, options[:context]).serializable_hash(options)
         end
       end

lib/active_model/serializer/adapter/json_api/link.rb

@@ -0,0 +1,34 @@
+module ActiveModel
+  class Serializer
+    module Adapter
+      class JsonApi
+        class Link
+          def initialize(serializer)
+            @object = serializer.object
+            @scope = serializer.scope
+          end
+
+          def href(value)
+            self._href = value
+          end
+
+          def meta(value)
+            self._meta = value
+          end
+
+          def to_hash
+            hash = { href: _href }
+            hash.merge!(meta: _meta) if _meta
+
+            hash
+          end
+
+          protected
+
+          attr_accessor :_href, :_meta
+          attr_reader :object, :scope
+        end
+      end
+    end
+  end
+end

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