I am using django to interface with an existing database that I have no control over.
The database frequently uses multiple columns for making relations and has uniqueness over all of those columns.

The django inspectdb model generator correctly identified these as unique_together = (('foo', 'bar'), ('foo', 'bar'),), resulting in the following model:

class A(models.Model):
    foo = models.IntegerField(db_column='FOO', primary_key=True)
    bar = models.IntegerField(db_column='BAR')

    class Meta:
        managed = False
        db_table = 'A'
        unique_together = (('foo', 'bar'), ('foo', 'bar'),)

Now there is another table whose entries relate to that table on both columns. When querying using SQL I would relate them like this:

SELECT * FROM A LEFT JOIN B ON A.foo = B.foo AND A.bar = B.bar

However inspectdb failed to correctly map the two columns being used in a relating table:

class B(models.Model):
    foo = models.OneToOneField(A, models.DO_NOTHING, db_column='A', primary_key=True)
    bar = models.ForeignKey(A, models.DO_NOTHING, db_column='A')

The above is what it generated, however in reality it is a many-to-one relationship to table A. This then causes an error at runtime because the column bar in table A is not unique.

Can I somehow define this relationship in a django model?
And how do I query this efficiently so that django generates a JOIN expression with two conditionals?

I am using django to interface with an existing database that I have no control over.
The database frequently uses multiple columns for making relations and has uniqueness over all of those columns.

The django inspectdb model generator correctly identified these as unique_together = (('foo', 'bar'), ('foo', 'bar'),), resulting in the following model:

class A(models.Model):
    foo = models.IntegerField(db_column='FOO', primary_key=True)
    bar = models.IntegerField(db_column='BAR')

    class Meta:
        managed = False
        db_table = 'A'
        unique_together = (('foo', 'bar'), ('foo', 'bar'),)

Now there is another table whose entries relate to that table on both columns. When querying using SQL I would relate them like this:

SELECT * FROM A LEFT JOIN B ON A.foo = B.foo AND A.bar = B.bar

However inspectdb failed to correctly map the two columns being used in a relating table:

class B(models.Model):
    foo = models.OneToOneField(A, models.DO_NOTHING, db_column='A', primary_key=True)
    bar = models.ForeignKey(A, models.DO_NOTHING, db_column='A')

The above is what it generated, however in reality it is a many-to-one relationship to table A. This then causes an error at runtime because the column bar in table A is not unique.

Can I somehow define this relationship in a django model?
And how do I query this efficiently so that django generates a JOIN expression with two conditionals?

• django

• No, Django does not allow keys to be composite ones, nor Foreign keys to link to multiple ones. – willeM_ Van Onsem Commented Jan 19 at 21:02

1 Answer 1

Composite keys cannot be related to in Django. Check the ForeignObject mention in that article however, but note the explicit warning given there.

You can probably create a dirty workaround to get your fetch working in a relatively straightforward way, but it won't retain the ORM's relationship management:

class B(models.Model):
    foo = models.IntegerField()
    bar = models.IntegerField()

b = B.objects.first()
a = A.objects.filter(foo=b.foo, bar=b.bar)