Composite primary key in django

Question

I have a legacy db table which has composite primary key. I don\'t think I will be able to change the structure to include a surrogate key, as there is some code written tha

Answer 1

Another option is to set managed=False in the model's Meta, then manually create the table.

class MyTable(models.Model):
    foo = models.IntegerField(primary_key=True)
    bar = models.IntegerField()
    baz = models.IntegerField()

    class Meta:
        managed = False
        db_table = 'myapp_mytable'

    def __repr__(self):
        return f'<MyTable: MyTable object ({self.foo}, {self.bar}, {self.baz)>'

In a postgres shell:

CREATE TABLE myapp_mytable (
    foo INTEGER NOT NULL,
    bar INTEGER NOT NULL,
    baz INTEGER NOT NULL,
    PRIMARY KEY(foo, bar, baz)
);

It appears to behave correctly:

>>> MyTable.objects.create(foo=1, bar=1, baz=1)
<MyTable: MyTable object (1, 1, 1)>

>>> MyTable.objects.create(foo=1, bar=1, baz=2)
<MyTable: MyTable object (1, 1, 2)>

>>> MyTable.objects.create(foo=1, bar=1, baz=2)
django.db.utils.IntegrityError: duplicate key value violates unique constraint "myapp_mytable_pkey"
DETAIL:  Key (foo, bar, baz)=(1, 1, 2) already exists.

Note that this is only tested in Django 3.x, so I'm not sure if it works in older versions.

Answer 2

The accepted answer is fine. However, it's a little old. unique_together may be deprecated in favor of UniqueConstraint. So, the better way of doing this would be;

UniqueConstraint(fields = ['key1', 'key2'], name = 'constraint_name')

Answer 3

Try similar below code:

class MyTable(models.Model):
    class Meta:
        unique_together = (('key1', 'key2'),)

    key1 = models.IntegerField(primary_key=True)
    key2 = models.IntegerField()

or if you want only unique mixed fields:

class MyTable(models.Model):
    class Meta:
        unique_together = (('key1', 'key2'),)

    key1 = models.IntegerField()
    key2 = models.IntegerField()

EDIT: I would like to note that there is a problem with this approach if there are 3 columns. Update queries don't work because it tries to update (puts pk fields right after "SET") the fields that are unique together and obviously fails.