update_or_create doesn't work with unique_together fields

[olsonap]

I have a model like this:

class MyModel(models.Model):
    field1 = models.IntegerField(null=False)
    field2 = models.IntegerField(null=False)
    field3 = models.TextField(null=False)
    field4 = models.TextField(null=False, primary_key=True)
    field5 = models.TextField(null=True)
    field6 = models.IntegerField(null=False)

    class Meta:
        managed = False
        unique_together = ('field2', 'field3', 'field4')

This is an unmanaged table with a clustered primary key.
The second, third, and fourth columns compose the primary key.

Using update_or_create() I can create new entries all day; HOWEVER, when I provide fields that match those unique_together fields, it updates the field1, field5, field6 columns (the non-unique together columns) for ALL rows where the primary key matches the value provided for that key.

I'm sure this is a great corner case. I can't add a new ID column to this table to use as the primary key, and none of my columns are actually unique on their own.