Error when issuing bulk_create with FK to non-primary-key rel field
So the `bulk_create()` method just pulls field data off each model and lines it up for insert. When the model being created has a foreign-key, then peewee is pulling off the related model instance. When doing the insert, we don't realize that we want the related model attr that corresponds to the FK and are just blindly grabbing the PK. Fixed by 2944cd21f76282306dc034432d4b9270553883a4
        u1 = BCUser.create(username='u1')
        u2 = BCUser.create(username='u2')
        BCTweet.bulk_create([
            BCTweet(user='u1', content='t%s' % i)
            for i in range(4)])

Throws a foreign-key mismatch error.