need to invoke a method on referenced collection before populate when using mongoose

[DavidHuang2002]

Before populating the referenced field with a collection, the referenced collection needed to be registered, currently, to do that, we run some find a method that has no consequence, otherwise, we can get a Model not registered error. It can be pretty confusing in code.

I think the cause of this problem is that the a schema is only registed to a model when we first import it and invoke a method on it, which might not have happened when we run populate. I think can be fixed by running all model registration statement(like the following) when the server first starts.