From 053eb1e213c7369cab4de683d7128a90b7a10089 Mon Sep 17 00:00:00 2001
From: Nicolas Delaby <nicolas@dryad.net>
Date: Tue, 12 Aug 2025 17:08:28 +0200
Subject: [PATCH 1/2] Condition of UniqueValidator can be read-only

We can't always expect to find the value of the condition in the serializer
if the field is read-only.
---
 rest_framework/validators.py |  7 ++++++-
 tests/test_validators.py     | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/rest_framework/validators.py b/rest_framework/validators.py
index 4c444cf01e..76d2a2159f 100644
--- a/rest_framework/validators.py
+++ b/rest_framework/validators.py
@@ -189,7 +189,12 @@ def __call__(self, attrs, serializer):
             ]
 
         condition_sources = (serializer.fields[field_name].source for field_name in self.condition_fields)
-        condition_kwargs = {source: attrs[source] for source in condition_sources}
+        condition_kwargs = {
+            source: attrs[source]
+            if source in attrs
+            else getattr(serializer.instance, source)
+            for source in condition_sources
+        }
         if checked_values and None not in checked_values and qs_exists_with_condition(queryset, self.condition, condition_kwargs):
             field_names = ', '.join(self.fields)
             message = self.message.format(field_names=field_names)
diff --git a/tests/test_validators.py b/tests/test_validators.py
index c594eecbe5..15f30f1c05 100644
--- a/tests/test_validators.py
+++ b/tests/test_validators.py
@@ -589,6 +589,21 @@ class Meta:
         ]
 
 
+class UniqueConstraintReadOnlyFieldModel(models.Model):
+    state = models.CharField(max_length=100, default="new")
+    position = models.IntegerField()
+    something = models.IntegerField()
+
+    class Meta:
+        constraints = [
+            models.UniqueConstraint(
+                name="unique_constraint_%(class)s",
+                fields=("position", "something"),
+                condition=models.Q(state="new"),
+            ),
+        ]
+
+
 class UniqueConstraintNullableModel(models.Model):
     title = models.CharField(max_length=100)
     age = models.IntegerField(null=True)
@@ -738,6 +753,23 @@ class Meta:
         )
         assert serializer.is_valid()
 
+    def test_uniq_constraint_condition_read_only(self):
+        class UniqueConstraintReadOnlyFieldModelSerializer(serializers.ModelSerializer):
+            class Meta:
+                model = UniqueConstraintReadOnlyFieldModel
+                read_only_fields = ("state",)
+                fields = ("position", "something", *read_only_fields)
+
+        serializer = UniqueConstraintReadOnlyFieldModelSerializer(
+            data={"position": 1, "something": 1}
+        )
+        assert serializer.is_valid()
+        UniqueConstraintReadOnlyFieldModel.objects.create(position=1, something=1)
+        serializer = UniqueConstraintReadOnlyFieldModelSerializer(
+            data={"position": 1, "something": 1}
+        )
+        assert not serializer.is_valid()
+
 
 # Tests for `UniqueForDateValidator`
 # ----------------------------------

From 9430c4982a6b556ff6c94ff357af9db540a6d9eb Mon Sep 17 00:00:00 2001
From: Nicolas Delaby <nicolas@dryad.net>
Date: Tue, 12 Aug 2025 18:30:38 +0200
Subject: [PATCH 2/2] Reproducible test

---
 tests/test_validators.py | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/tests/test_validators.py b/tests/test_validators.py
index 15f30f1c05..ea5bf3a4dd 100644
--- a/tests/test_validators.py
+++ b/tests/test_validators.py
@@ -753,22 +753,30 @@ class Meta:
         )
         assert serializer.is_valid()
 
-    def test_uniq_constraint_condition_read_only(self):
+    def test_uniq_constraint_condition_read_only_create(self):
         class UniqueConstraintReadOnlyFieldModelSerializer(serializers.ModelSerializer):
             class Meta:
                 model = UniqueConstraintReadOnlyFieldModel
                 read_only_fields = ("state",)
                 fields = ("position", "something", *read_only_fields)
-
         serializer = UniqueConstraintReadOnlyFieldModelSerializer(
             data={"position": 1, "something": 1}
         )
         assert serializer.is_valid()
-        UniqueConstraintReadOnlyFieldModel.objects.create(position=1, something=1)
+
+    def test_uniq_constraint_condition_read_only_partial(self):
+        class UniqueConstraintReadOnlyFieldModelSerializer(serializers.ModelSerializer):
+            class Meta:
+                model = UniqueConstraintReadOnlyFieldModel
+                read_only_fields = ("state",)
+                fields = ("position", "something", *read_only_fields)
+        instance = UniqueConstraintReadOnlyFieldModel.objects.create(position=1, something=1)
         serializer = UniqueConstraintReadOnlyFieldModelSerializer(
-            data={"position": 1, "something": 1}
+            instance=instance,
+            data={"position": 1, "something": 1},
+            partial=True
         )
-        assert not serializer.is_valid()
+        assert serializer.is_valid()
 
 
 # Tests for `UniqueForDateValidator`