and fk.rel.to is not contained in result of the call to the method parent_model._meta.get_parent_list. raise an ValueError with an argument string "fk_name '%s' is not a ForeignKey to '%s.%'.", where '%s' is replaced with,
raise ValueError ( "fk_name '%s' is not a ForeignKey to '%s.%'." % ( fk_name , parent_model . _meta . app_label , parent_model . _meta . object_name ) )