def only(self, *fields):
        if fields == (None,):
            # Can only pass None to defer(), not only(), as the rest option.
            # That won't stop people trying to do this, so let's be explicit.
            raise TypeError("Cannot pass None as an argument to only().")
        clone = self._clone()
        clone.query.add_immediate_loading(fields)
        return clone
Essentially, the opposite of defer. Only the fields passed into this
dataset/ETHPy150Open AppScale/appscale/AppServer/lib/django-1.5/django/db/models/query.py/QuerySet.only