From 8de6a6ca60ade50360e3065369940ea375bde421 Mon Sep 17 00:00:00 2001
From: Sergei Shishov <sshishov.sshishov@gmail.com>
Date: Sat, 2 Aug 2025 15:45:34 +0300
Subject: [PATCH] fix(blank/unique): add `default` handling for `blank=True`
 field

Signed-off-by: Sergei Shishov <sshishov.sshishov@gmail.com>
---
 rest_framework/serializers.py |  2 ++
 tests/test_validators.py      | 66 +++++++++++++++++++++++++++++++++++
 2 files changed, 68 insertions(+)

diff --git a/rest_framework/serializers.py b/rest_framework/serializers.py
index 8fe284bc8..f23d663cc 100644
--- a/rest_framework/serializers.py
+++ b/rest_framework/serializers.py
@@ -1496,6 +1496,8 @@ class ModelSerializer(Serializer):
                 default = unique_constraint_field.default
             elif unique_constraint_field.null:
                 default = None
+            elif unique_constraint_field.blank:
+                default = ''
             else:
                 default = empty
 
diff --git a/tests/test_validators.py b/tests/test_validators.py
index c594eecbe..8dda7f3a9 100644
--- a/tests/test_validators.py
+++ b/tests/test_validators.py
@@ -149,6 +149,14 @@ class UniquenessTogetherModel(models.Model):
         unique_together = ('race_name', 'position')
 
 
+class BlankUniquenessTogetherModel(models.Model):
+    race_name = models.CharField(max_length=100, blank=True)
+    position = models.IntegerField()
+
+    class Meta:
+        unique_together = ('race_name', 'position')
+
+
 class NullUniquenessTogetherModel(models.Model):
     """
     Used to ensure that null values are not included when checking
@@ -176,6 +184,12 @@ class UniquenessTogetherSerializer(serializers.ModelSerializer):
         fields = '__all__'
 
 
+class BlankUniquenessTogetherSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = BlankUniquenessTogetherModel
+        fields = '__all__'
+
+
 class NullUniquenessTogetherSerializer(serializers.ModelSerializer):
     class Meta:
         model = NullUniquenessTogetherModel
@@ -461,6 +475,34 @@ class TestUniquenessTogetherValidation(TestCase):
         serializer = NullUniquenessTogetherSerializer(data=data)
         assert not serializer.is_valid()
 
+    def test_validation_for_provided_blank_fields(self):
+        BlankUniquenessTogetherModel.objects.create(
+            position=1
+        )
+        data = {
+            'race_name': '',
+            'position': 1
+        }
+        serializer = BlankUniquenessTogetherSerializer(data=data)
+        assert not serializer.is_valid()
+
+    def test_validation_for_missing_blank_fields(self):
+        BlankUniquenessTogetherModel.objects.create(
+            position=1
+        )
+        data = {
+            'position': 1
+        }
+        serializer = BlankUniquenessTogetherSerializer(data=data)
+        assert not serializer.is_valid()
+
+    def test_ignore_validation_for_missing_blank_fields(self):
+        data = {
+            'position': 1
+        }
+        serializer = BlankUniquenessTogetherSerializer(data=data)
+        assert serializer.is_valid(), serializer.errors
+
     def test_ignore_validation_for_unchanged_fields(self):
         """
         If all fields in the unique together constraint are unchanged,
@@ -589,6 +631,18 @@ class UniqueConstraintModel(models.Model):
         ]
 
 
+class UniqueConstraintBlankModel(models.Model):
+    title = models.CharField(max_length=100, blank=True)
+    age = models.IntegerField()
+    tag = models.CharField(max_length=100, blank=True)
+
+    class Meta:
+        constraints = [
+            # Unique constraint on 2 blank fields
+            models.UniqueConstraint(name='unique_constraint', fields=('age', 'tag'), condition=~models.Q(models.Q(title='') & models.Q(tag='True')))
+        ]
+
+
 class UniqueConstraintNullableModel(models.Model):
     title = models.CharField(max_length=100)
     age = models.IntegerField(null=True)
@@ -607,6 +661,12 @@ class UniqueConstraintSerializer(serializers.ModelSerializer):
         fields = '__all__'
 
 
+class UniqueConstraintBlankSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = UniqueConstraintBlankModel
+        fields = ('title', 'age', 'tag')
+
+
 class UniqueConstraintNullableSerializer(serializers.ModelSerializer):
     class Meta:
         model = UniqueConstraintNullableModel
@@ -714,6 +774,12 @@ class TestUniqueConstraintValidation(TestCase):
         ids_in_qs = {frozenset(v.queryset.values_list(flat=True)) for v in validators if hasattr(v, "queryset")}
         assert ids_in_qs == {frozenset([1]), frozenset([3])}
 
+    def test_blank_uqnique_constraint_fields_are_not_required(self):
+        serializer = UniqueConstraintBlankSerializer(data={'age': 25})
+        self.assertTrue(serializer.is_valid(), serializer.errors)
+        result = serializer.save()
+        self.assertIsInstance(result, UniqueConstraintBlankModel)
+
     def test_nullable_unique_constraint_fields_are_not_required(self):
         serializer = UniqueConstraintNullableSerializer(data={'title': 'Bob'})
         self.assertTrue(serializer.is_valid(), serializer.errors)