Declare api relation relations as nested resources

app/controllers/api/relations/relations_controller.rb

@@ -0,0 +1,25 @@
+module Api
+  module Relations
+    class RelationsController < ApiController
+      authorize_resource
+
+      before_action :set_request_formats
+
+      def index
+        relation_ids = RelationMember.where(:member_type => "Relation", :member_id => params[:relation_id]).collect(&:relation_id).uniq
+
+        @relations = []
+
+        Relation.find(relation_ids).each do |relation|
+          @relations << relation if relation.visible
+        end
+
+        # Render the result
+        respond_to do |format|
+          format.xml
+          format.json
+        end
+      end
+    end
+  end
+end