Multiple User Models

[JanStevens]

Hello,

I have a centralized user application with doorkeeper and devise. There are two user tables in this application. Tenant specific users and Admin users that can manage the tenant specific users.

Both should be able to authenticate client apps. I see that the resource_owner_id is set in the access_grants and access_tokens.

The problem is that for multiple user models you get conflicts because my admin user has id = 1 and my tenant user has id = 1 and they are both registered for the same client app.

the solution is to make the resource_owner_id polymorphic so multiple user models can be used or at least a resource_owner_type should be set.

[JanStevens]

PR #461 Adds this feature