Django 3.16 not checking DEFERRED unique constraints

[Enorio]

I was using the rest-framework version 3.14 and have the following code:

class A(models.Model):
    ....
    class Meta:
        constraints = [
            UniqueConstraint(
                name="unique_one",
                fields=['field_a', 'field_b'],
                deferrable=Deferrable.IMMEDIATE
            ),
            UniqueConstraint(
                name='unique_two',
                fields=['field_a', 'sort_index'],
                deferrable=Deferrable.DEFERRED
            )
        ]

class ASerializer(serializers.ModelSerializer):
    class Meta:
        model = A
        fields = '__all__'
        validators = [
            UniqueTogetherValidator(
                queryset=A.objects.all(),
                fields=['field_a', 'field_b']
            )
        ]


class B(models.Model):
    ...
    class Meta:
        constraints = [
            UniqueConstraint(
                name='unique_one',
                fields=['field_a', 'sort_index'],
                deferrable=Deferrable.DEFERRED,
            )
        ]

class BSerializer(serializers.ModelSerializer):
    class Meta:
        model = B
        fields = '__all__'

My code basically has a sort_index field, that is the position of a certain A or B.
For example, if I have a list of Bs, adding a new one updates all the sort_index, following some rules:

list_of_b = [
    B(name="A", field_a=1, sort_index=1),
    B(name="B", field_a=1, sort_index=2)
]
b_serializer = BSerializer(data={"name":"C", "sort_index":1})
b_serializer.save()

# final list of Bs
list_of_b = [
    B(name="C", field_a=1, sort_index=1),
    B(name="A", field_a=1, sort_index=2),
    B(name="B", field_a=1, sort_index=3)
]

Now, with version 3.16, this raises the error {'non_field_errors': [ErrorDetail(string='The fields field_a, sort_index must make a unique set.', code='unique')]}
This error does not occur in the ASerializer, I'm guessing because I override the constraints with the validators?

Should I add validators = [] to override it, or is some configuration that uses the Deferred?

[sanyamk23]

Hey 👋 — thanks for bringing this up!

This behavior is actually related to a change introduced in Django REST Framework 3.15, where UniqueConstraints from your model’s Meta.constraints are now automatically converted into serializer-level validators.

However, there's a subtle but important caveat here: DRF's validators run at the Python layer before your data ever reaches the database. So even if your database-level constraint is marked as Deferrable.DEFERRED, DRF doesn't know about it — and will still perform the uniqueness check right away. That’s why you're seeing the non_field_errors validation even though the DB constraint should allow it.

✅ How to fix it
To let your PostgreSQL constraint handle uniqueness deferral properly, you’ll need to explicitly override DRF’s auto-inferred validators in your serializer:

class BSerializer(serializers.ModelSerializer):
class Meta:
model = B
fields = 'all'
validators = [] # disables DRF’s automatic uniqueness validation

This skips DRF’s validation and lets your DB constraint (which supports Deferrable.DEFERRED) do its job during the transaction lifecycle — just like you intended.

📌 Suggestion for DRF
This could be a good note to add to the documentation! Something like:

“DRF does not currently support PostgreSQL’s deferrable constraints (Deferrable.DEFERRED). If you’re relying on deferred uniqueness checks, you should disable auto-inferred validators in your serializer using validators = [].”

I’d be happy to help put together a quick docs PR if this is something the maintainers are open to. Let me know!

Hope this helps clarify things 🙌