David Kimura PRO said over 6 years ago on Polymorphic Associations :

My guess would be when the type is saved to the database, it is being saved as Authority instead of Wobauth::Authority. Try adding a class_name to the association to see if that makes a difference. The users might be wobauth_users depending where it is declared.

has_many :users, class_name: 'Wobauth::Authority'