I've got two tables that I need to relate on a single field po_num. The data is imported from another source so while I have a little bit of control over what the tables look like but I can't change them too much. What I want to do is relate these two models so I can look up one from the other based on the po_num fields.

What I really need to do is join the two tables so I can do a where on a count of the related table. I would like to do filter for all Order objects that have 0 related EDI856 objects.

I tried adding a foreign key to the Order model and specified the db_column and to_fields as po_num but django didn't like that the fact that Edi856.po_num wasn't unique.

Here are the important fields of my current models that let me display but not filter for the data that I want.

class Edi856(models.Model):
    po_num = models.CharField(max_length=90, db_index=True )

class Order(models.Model):
    po_num = models.CharField(max_length=90, db_index=True) 
    def in_edi(self):
        '''Has the edi been processed?'''
        return Edi856.objects.filter(po_num = self.po_num).count()

Thanks for taking the time to read about my problem. I'm not sure what to do from here.