Modify ObtainAuthToken to use the User model's USERNAME_FIELD and password for authentication instead of assuming username and password.

[partho-debnath]

Summary

Django’s default user model uses username and password for authentication. However, when a custom user model is defined with USERNAME_FIELD = 'email', Django correctly uses email and password for authentication.

The issue was that Django REST Framework's obtain_auth_token endpoint (used for TokenAuthentication) still expected username and password, even when a custom user model used email instead of username.

Fix

This update modifies the ObtainAuthToken view to dynamically use USERNAME_FIELD instead of assuming username. Now, authentication works consistently, whether using the built-in User model or a custom one, with no inconsistencies.

Changes Made:

1. ObtainAuthToken now retrieves the USERNAME_FIELD from the user model.
2. It authenticates using USERNAME_FIELD and password, ensuring compatibility with both built-in and custom user models.

Impact

1. Users with a custom User model (e.g., USERNAME_FIELD = 'email') can now log in using email and password instead of username and password.
2. Users with the default User model or a custom User model where USERNAME_FIELD = 'username' can continue logging in using username and password as expected.

[partho-debnath]

If any changes are required, please let me know.

[browniebroke]

I'm not sure this falls under our current maintenance policy:

At this point in its lifespan we consider Django REST framework to be feature-complete. We focus on pull requests that track the continued development of Django versions, and generally do not accept new features or code formatting changes.

One could argue that this improves compatibility with custom Django user models, but on the other hand USERNAME_FIELD has been in Django for so long that it's a bit late to add this now...

It's also simple to customise in user-land right now, and this is explained in our docs.

If we were to ever accept it, we would need some tests to cover the behaviour with a customised user model...

[partho-debnath]

Thank you for reviewing my pull request and for your feedback. I understand that Django REST Framework is considered feature-complete and that new features are usually only accepted if they align with Django’s ongoing development.

However, I see this change as more of a compatibility improvement rather than a new feature. While USERNAME_FIELD has been in Django for a long time, ObtainAuthToken still assumes a username by default. This creates an inconsistency for projects that take advantage of Django’s built-in flexibility for custom user models.

@browniebroke, Are you saying that it won't be possible to accept or merge this change?