Forward method calls to parent resource when method missing in relationship

lib/jsonapi/serializable/relationship.rb

@@ -45,6 +45,18 @@ def linkage_data
 
         resources.respond_to?(:each) ? linkage_data : linkage_data.first
       end
+
+      def respond_to_missing?(m, include_private = false)
+        @_options[:_resource].respond_to?(m) || super
+      end
+
+      def method_missing(m, *args, &block)
+        if @_options[:_resource].respond_to?(m, true)
+          @_options[:_resource].send(m, *args, &block)
+        else
+          super
+        end
+      end
     end
   end
 end

lib/jsonapi/serializable/resource.rb

@@ -28,6 +28,7 @@ def initialize(exposures = {})
         @_relationships = self.class.relationship_blocks
                               .each_with_object({}) do |(k, v), h|
           opts = self.class.relationship_options[k] || {}
+          opts = opts.merge(_resource: self)
           h[k] = Relationship.new(@_exposures, opts, &v)
         end
         @_meta = if (b = self.class.meta_block)

spec/resource/relationship_spec.rb

@@ -1,8 +1,11 @@
 require 'spec_helper'
 
 describe JSONAPI::Serializable::Resource, '.relationship' do
-  let(:posts) { [Post.new(id: 1), Post.new(id: 2)] }
-  let(:user) { User.new(id: 'foo', posts: posts) }
+  let(:posts) do
+    [Post.new(id: 1, title: 'Post 1'),
+     Post.new(id: 2, title: 'Post 2')]
+  end
+  let(:user) { User.new(id: 'foo', name: 'Lucas', posts: posts) }
 
   it 'forwards to @object by default' do
     klass = Class.new(JSONAPI::Serializable::Resource) do
@@ -109,4 +112,27 @@
 
     expect(actual).to eq(expected)
   end
+
+  it 'forwards missing methods to parent resource if available' do
+    klass = Class.new(JSONAPI::Serializable::Resource) do
+      type 'users'
+
+      attribute(:name) { "#{@object.name} - writer of #{primary_post.title}" }
+
+      has_one :primary_post do
+        data { primary_post }
+      end
+
+      private
+
+      def primary_post
+        @object.posts.first
+      end
+    end
+
+    resource = klass.new(object: user, _class: { Post: SerializablePost })
+    actual = resource.as_jsonapi(include: [:primary_post])
+    expect(actual[:attributes][:name]).to eq('Lucas - writer of Post 1')
+    expect(actual[:relationships][:primary_post][:data][:id]).to eq('1')
+  end
 end