Just diving into pylons here, and am trying to get my head around the basics of SQLALchemy. I have figured out how to load a record by id:

user_q = session.query(model.User)
user = user_q.get(user_id)

But how do I query by a specific field (i.e. username)? I assume there is a quick way to do it with the model rather than hand-building the query. I think it has something with the add_column() function on the query object, but I can't quite figure out how to use it. I've been trying stuff like this, but obviously it doesn't work:

user_q = meta.Session.query(model.User).add_column('username'=user_name)
user = user_q.get()