Undocumented Change in Behavior Involving UniqueConstraint Fields

[Sandy Suh]

So there's a change between v3.14.0 and v3.15.1 involving serializer fields that are part of a UniqueConstraint, and this change isn't documented to my knowledge. Consider the following example:

class Dummy(models.Model):

    name = models.TextField(blank=True)

    url = models.URLField()

    class Meta:

        constraints = [models.UniqueConstraint(fields=["url", "name"], name="url_name_unique")]

class DummySerializer(serializers.ModelSerializer):

    class Meta:

        model = Dummy

        fields = ("name", "url")

In DRF 3.14.0, creating a Dummy using a DummySerializer without specifying a name will work (the name will be the empty string). But in DRF 3.15.1, this will not work. You'll get the error 'This field is required.' for the name field, and looking at the codebase, this seems to be due to changes to `get_uniqueness_extra_args` in serializers.py, which make a field that is part of a UniqueConstraint required by default.

To my knowledge, this change in behavior is not documented, so I thought I'd bring it to people's attention. 

Thanks!