[Answer]-Tastypie – Should method be in UserResource or ThreadResource?

From your question

returns all users participating in a thread.

so basically, your resource is User, hence the method should definitely be in the UserResource class.

The fact that you are only retrieving users for a certain thread is related to filtering: the operation shouldn’t query a thread instance to retrieve its users, but it should take some parameter (such as thread id) and query all the users, filtering by that id.

Of course, the implementation should be mostly influenced by your app’s needs.

I would also suggest to take a look at nested resources, perhaps this suits your needs better.