From 0327be56c27fb51eac9265840d04edc0f0f1e294 Mon Sep 17 00:00:00 2001
From: Carlton Gibson <carlton.gibson@noumenal.es>
Date: Fri, 6 Apr 2018 09:15:48 +0200
Subject: [PATCH 1/2] Add test for read_only + default unique_together
 validation.

---
 tests/test_validators.py | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/tests/test_validators.py b/tests/test_validators.py
index 62126ddb33..4bbddb64ba 100644
--- a/tests/test_validators.py
+++ b/tests/test_validators.py
@@ -277,6 +277,30 @@ class Meta:
         """)
         assert repr(serializer) == expected
 
+    def test_read_only_fields_with_default(self):
+        """
+        Special case of read_only + default DOES validate unique_together.
+        """
+        class ReadOnlyFieldWithDefaultSerializer(serializers.ModelSerializer):
+            race_name = serializers.CharField(max_length=100, read_only=True, default='example')
+
+            class Meta:
+                model = UniquenessTogetherModel
+                fields = ('id', 'race_name', 'position')
+
+        data = {'position': 2}
+        serializer = ReadOnlyFieldWithDefaultSerializer(data=data)
+
+        assert len(serializer.validators) == 1
+        assert isinstance(serializer.validators[0], UniqueTogetherValidator)
+        assert serializer.validators[0].fields == ('race_name', 'position')
+        assert not serializer.is_valid()
+        assert serializer.errors == {
+            'non_field_errors': [
+                'The fields race_name, position must make a unique set.'
+            ]
+        }
+
     def test_allow_explict_override(self):
         """
         Ensure validators can be explicitly removed..

From 2227bc47f8b287b66775948ffb60b2d9378ac84f Mon Sep 17 00:00:00 2001
From: Carlton Gibson <carlton.gibson@noumenal.es>
Date: Fri, 6 Apr 2018 12:15:30 +0200
Subject: [PATCH 2/2] Fix read_only + default validation

---
 rest_framework/serializers.py | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/rest_framework/serializers.py b/rest_framework/serializers.py
index d773902e89..7e84372def 100644
--- a/rest_framework/serializers.py
+++ b/rest_framework/serializers.py
@@ -441,6 +441,30 @@ def run_validation(self, data=empty):
 
         return value
 
+    def _read_only_defaults(self):
+        fields = [
+            field for field in self.fields.values()
+            if (field.read_only) and (field.default != empty) and (field.source != '*') and ('.' not in field.source)
+        ]
+
+        defaults = OrderedDict()
+        for field in fields:
+            try:
+                default = field.get_default()
+            except SkipField:
+                continue
+            defaults[field.field_name] = default
+
+        return defaults
+
+    def run_validators(self, value):
+        """
+        Add read_only fields with defaults to value before running validators.
+        """
+        to_validate = self._read_only_defaults()
+        to_validate.update(value)
+        super(Serializer, self).run_validators(to_validate)
+
     def to_internal_value(self, data):
         """
         Dict of native values <- Dict of primitive datatypes.
@@ -1477,6 +1501,12 @@ def get_unique_together_validators(self):
             if (field.source != '*') and ('.' not in field.source)
         }
 
+        # Special Case: Add read_only fields with defaults.
+        field_names |= {
+            field.source for field in self.fields.values()
+            if (field.read_only) and (field.default != empty) and (field.source != '*') and ('.' not in field.source)
+        }
+
         # Note that we make sure to check `unique_together` both on the
         # base model class, but also on any parent classes.
         validators = []