@rodrigoarino remember that Rails  automatically infers the class name from the association name, if you create a object of this and do intent ```my_appointment.owner``` you will get ```NameError: uninitialized constant Appointment::Owner```
The correct form is: 
```
   belongs_to :user
   belongs_to :property
   belongs_to :customer
   belongs_to :owner, class_name: "User" 
```