Class f.d.Ref(object):

This allows a DataSet to reference an id column of a "foreign key" DataSet before it exists.

Ref is a Descriptor containing a deferred value to an attribute of a data object (like an instance of a SQLAlchemy mapped class). It provides the DataSet a way to cloak the fact that "id" is an attribute only populated after said data object is saved to the database. In other words, the DataSet doesn't know or care when it has been loaded or not. It thinks it is referencing "id" all the same. The timing of when id is accessed is handled by the LoadableFixture.