migration.py

class CourseStudentsJoin(models.Model):

    """Creates a mock of the previous many to many join."""

    class Meta:
        db_table = u'course_course_students'

    course = models.ForeignKey(Course)
    student = models.ForeignKey(User)
    
class Migration(SchemaMigration):

    def forwards(self, orm):
        # Adding model 'CourseMembership'
        db.create_table(u'course_coursemembership', (
            (u'id', self.gf('django.db.models.fields.AutoField')(primary_key=True)),
            ('course', self.gf('django.db.models.fields.related.ForeignKey')(to=orm['course.Course'])),
            ('student', self.gf('django.db.models.fields.related.ForeignKey')(to=orm['auth.User'])),
        ))
        db.send_create_signal(u'course', ['CourseMembership'])

        # Adding unique constraint on 'CourseMembership', fields ['course', 'student']
        db.create_unique(u'course_coursemembership', ['course_id', 'student_id'])

        # Removing M2M table for field students on 'Course'
        db.delete_table(db.shorten_name(u'course_course_students'))

        # CUSTOM CODE
        # this is needed in order to convert all existing joins to new joins using the
        # CourseMembership model.

        CourseMembership.objects.bulk_create([
            CourseMembership(
                student=c.student,
                course=c.course
            )
            for c in CourseStudentsJoin.objects.all()
        ])


    def backwards(self, orm):
        # Removing unique constraint on 'CourseMembership', fields ['course', 'student']
        db.delete_unique(u'course_coursemembership', ['course_id', 'student_id'])

        # Deleting model 'CourseMembership'
        db.delete_table(u'course_coursemembership')

        # Adding M2M table for field students on 'Course'
        m2m_table_name = db.shorten_name(u'course_course_students')
        db.create_table(m2m_table_name, (
            ('id', models.AutoField(verbose_name='ID', primary_key=True, auto_created=True)),
            ('course', models.ForeignKey(orm[u'course.course'], null=False)),
            ('user', models.ForeignKey(orm[u'auth.user'], null=False))
        ))
        db.create_unique(m2m_table_name, ['course_id', 'user_id'])

        # CUSTOM CODE
        # convert all the new CourseMembership objects back to the previous join.
        ?