DJANGO > 2.2 - Unique together on foreign keys A, B OR foreign keys A, B, C

Question

In Django 2.2, you can get rid of unique_together in favor of a constraints list with a UniqueConstraint class that can be handy because it has a condition arg.

MyObject has three foreign keys, one is optional.

class MyObject(ModelBase):
    parent=... # mandatory
    source=... # mandatory
    subsource=... # optional

    class Meta:
        constraints = [
            models.UniqueConstraint(
                fields=["parent", "source", "subsource"],    
                name="unique_subsource"
            ),
            models.UniqueConstraint(
                fields=["parent", "source"],
                condition=models.Q(subsource=None),
                name="unique_source",
            ),
        ]

Imagine I try to create successively the following objects and the expected validation:

parent   source   subsource   valid?
1.       1.       1.          yes
1.       1.       2.          yes
1.       1.       -           yes
1.       1.       -           no
1.       1.       3.          yes
1.       2.       -           yes
...

So I wrote two tests:

def test_unique1(self):
    """ unique parent/source/subsource """
    parent = ParentFactory()
    source = SourceFactory()
    subsource = SubsourceFactory()
    MyObjectFactory(parent=parent, source=source, subsource=subsource)
    myobj = MyObjectFactory.build(parent=parent, source=source, subsource=subsource)
    self.should_raise_validation_error(myobj)

def test_unique2(self):
    """ unique parent/source """
    parent = ParentFactory()
    source = SourceFactory()
    subsource = SubsourceFactory()
    MyObjectFactory(parent=parent, source=source)
    myobj = MyObjectFactory.build(parent=parent, source=source)
    self.should_raise_validation_error(myobj)

But the latter does not raise any validation error:

.....F.
======================================================================
FAIL: test_unique2
unique parent/source/subsource
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/sebastiennicolaidis/dev/python/scientcap/macrovars/tests/models/test_dsflink.py", line 50, in test_unique2
    self.should_raise_validation_error(dsflink)
  File "/Users/sebastiennicolaidis/dev/python/scientcap/shared/tests/base_model_test.py", line 9, in should_raise_validation_error
    instance.full_clean()
AssertionError: ValidationError not raised

----------------------------------------------------------------------
Ran 7 tests in 0.141s

[UPDATE 2] output of the sqlmigrate:

BEGIN;
--
-- Change Meta options on dataset
--
--
-- Change Meta options on dsforecast
--
--
-- Alter unique_together for dsflink (0 constraint(s))
--
ALTER TABLE "macrovars_dsflink" DROP CONSTRAINT "macrovars_dsflink_parent_id_source_id_73901780_uniq";
--
-- Create constraint unique_subsource on model dsflink
--
CREATE UNIQUE INDEX "unique_subsource" ON "macrovars_dsflink" ("parent_id", "source_id", "subsource_id") WHERE "subsource_id" IS NOT NULL;
--
-- Create constraint unique_source on model dsflink
--
CREATE UNIQUE INDEX "unique_source" ON "macrovars_dsflink" ("parent_id", "source_id") WHERE "subsource_id" IS NULL;
COMMIT;

Answer 1

I think the solution would be to create both the constraint with condition.

class MyObject(ModelBase):
    parent=... # mandatory
    source=... # mandatory
    subsource=... # optional

    class Meta:
        constraints = [
            models.UniqueConstraint(
                fields=["parent", "source", "subsource"],    
                condition=models.Q(subsource__isnull=False),  # <- filter subsource is null
                name="unique_subsource"
            ),
            models.UniqueConstraint(
                fields=["parent", "source"],
                condition=models.Q(subsource__isnull=True),
                name="unique_source",
            ),
        ]