Allow resource level links to be defined

Graphiti supports links at a relationship level and at the top-level of the
jsonapi document.

Relationship level links are mainly to support lazy loading, and top level links
are used solely for pagination.

This commit adds support for defining links at the resource level.

These links could be used to provide the URL to the resource, or alternative
representation of the resource, or even actions to perform against the resource.

{
  "data": {
    "id": 1,
    "type": "tests",
    "attributes": {},
    "relationships": {},
    "links": {
      "self": "http://test.com/api/v1/tests/1"
    }
  }
}

To define a link on a resource:

link :self do |model|
  "#{self.endpoint[:url]}/#{model.id}"
end

Author: superslau

lib/graphiti/resource.rb

@@ -148,5 +148,16 @@ def transaction
       end
       response
     end
+
+    def links?
+      self.class.links.any?
+    end
+
+    def links(model)
+      self.class.links.inject({}) do |memo, (name, blk)| 
+        memo[name] = instance_exec(model, &blk)
+        memo
+      end
+    end
   end
 end

lib/graphiti/resource/configuration.rb

@@ -198,6 +198,7 @@ def config
               extra_attributes: {},
               sideloads: {},
               callbacks: {},
+              links: {},
             }
         end
 
@@ -236,6 +237,10 @@ def pagination
         def default_filters
           config[:default_filters]
         end
+
+        def links
+          config[:links]
+        end
       end
 
       def get_attr!(name, flag, options = {})

lib/graphiti/resource/dsl.rb

@@ -146,6 +146,10 @@ def on_extra_attribute(name, &blk)
           end
         end
 
+        def link(name, &blk)
+          config[:links][name.to_sym] = blk
+        end
+
         def all_attributes
           attributes.merge(extra_attributes)
         end

lib/graphiti/serializer.rb

@@ -25,6 +25,7 @@ def self.inherited(klass)
     def as_jsonapi(*)
       super.tap do |hash|
         strip_relationships!(hash) if strip_relationships?
+        hash[:links] = @resource.links(@object) if @resource.links?
       end
     end