Allow relationship links to be declared as object method (Netflix#2)

* Allow relationship links to be declared as object method

* Relationship links note added to readme

Author: jopotts

README.md

@@ -278,6 +278,12 @@ class MovieSerializer
 end
 ```
 
+Relationship links can also be configured to be defined as a method on the object.
+
+```ruby
+  has_many :actors, links: :actor_relationship_links
+```
+
 This will create a `self` reference for the relationship, and a `related` link for loading the actors relationship later. NB: This will not automatically disable loading the data in the relationship, you'll need to do that using the `lazy_load_data` option:
 
 ```ruby

lib/fast_jsonapi/relationship.rb

@@ -104,8 +104,12 @@ def fetch_id(record, params)
     end
 
     def add_links_hash(record, params, output_hash)
-      output_hash[key][:links] = links.each_with_object({}) do |(key, method), hash|
-        Link.new(key: key, method: method).serialize(record, params, hash)\
+      if links.is_a?(Symbol)
+        output_hash[key][:links] = record.public_send(links)
+      else
+        output_hash[key][:links] = links.each_with_object({}) do |(key, method), hash|
+          Link.new(key: key, method: method).serialize(record, params, hash)\
+        end
       end
     end
 

spec/lib/object_serializer_relationship_links_spec.rb

@@ -67,5 +67,28 @@ class LazyLoadingMovieSerializer < MovieSerializer
         expect(actor_hash).not_to have_key(:data)
       end
     end
+
+    context "relationship links defined by a method on the object" do
+      before(:context) do
+        class Movie
+          def relationship_links
+            { self: "http://movies.com/#{id}/relationships/actors" }
+          end
+        end
+
+        class LinksPassingMovieSerializer < MovieSerializer
+          has_many :actors, links: :relationship_links
+        end
+      end
+
+      let(:serializer) { LinksPassingMovieSerializer.new(movie) }
+      let(:links) { hash[:data][:relationships][:actors][:links] }
+      let(:relationship_url) { "http://movies.com/#{movie.id}/relationships/actors" }
+
+      it "generates relationship links in the object" do
+        expect(links).to be_present
+        expect(links[:self]).to eq(relationship_url)
+      end
+    end
   end
 end