add missing file

app/models/preview_association.rb

@@ -0,0 +1,16 @@
+class PreviewAssociation
+
+  include Mongoid::Document
+  include Mongoid::Timestamps
+  
+  field :object, :type=> Hash
+  field :field_name_for_rebuild
+  field :object_in_array , :type => Boolean,default: false
+
+  # field :to_save, :type => Boolean
+  field :should_destroy, :type => Boolean
+  
+  belongs_to :preview
+  # embedded_in :news_bulletin
+
+end