[3.0.x] Fixed #31071 -- Disabled insert optimization for primary keys with defaults when loading fixtures.

Model.save_base() is called directly when loading fixtures and assumes
existing rows will be updated. Branching of "raw" allows to maintain
the optimization introduced in #29260 while supporting this edge case.

Regression in 85458e94e38c20e57939947ee515a1a53689659f.

Thanks Reupen Shah for the report.

Backport of 5779cc938a34eb96815c7a40ded2c8f6c8087c58 from master

4 files changed, 13 insertions, 1 deletions

diff --git a/django/db/models/base.py b/django/db/models/base.py
index 0b8425aa85..3552dc5783 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -848,6 +848,7 @@ class Model(metaclass=ModelBase):
         updated = False
         # Skip an UPDATE when adding an instance and primary key has a default.
         if (
+            not raw and
             not force_insert and
             self._state.adding and
             self._meta.pk.default and
diff --git a/docs/releases/3.0.2.txt b/docs/releases/3.0.2.txt
index ebb85d1f46..e232fa81f8 100644
--- a/docs/releases/3.0.2.txt
+++ b/docs/releases/3.0.2.txt
@@ -18,3 +18,7 @@ Bugfixes
 * Fixed a regression in Django 3.0 that caused a migration crash on PostgreSQL
   10+ when adding a foreign key and changing data in the same migration
   (:ticket:`31106`).
+
+* Fixed a regression in Django 3.0 where loading fixtures crashed for models
+  defining a :attr:`~django.db.models.Field.default` for the primary key
+  (:ticket:`31071`).
diff --git a/tests/serializers/models/data.py b/tests/serializers/models/data.py
index 533ccf6830..3a55b89c9b 100644
--- a/tests/serializers/models/data.py
+++ b/tests/serializers/models/data.py
@@ -4,6 +4,8 @@ The following classes are for testing basic data marshalling, including
 NULL values, where allowed.
 The basic idea is to have a model for each Django data type.
 """
+import uuid
+
 from django.contrib.contenttypes.fields import (
     GenericForeignKey, GenericRelation,
 )
@@ -253,6 +255,10 @@ class UUIDData(models.Model):
     data = models.UUIDField(primary_key=True)
 
 
+class UUIDDefaultData(models.Model):
+    data = models.UUIDField(primary_key=True, default=uuid.uuid4)
+
+
 class FKToUUID(models.Model):
     data = models.ForeignKey(UUIDData, models.CASCADE)
 
diff --git a/tests/serializers/test_data.py b/tests/serializers/test_data.py
index 9f0e311d62..0fcde35bd3 100644
--- a/tests/serializers/test_data.py
+++ b/tests/serializers/test_data.py
@@ -26,7 +26,7 @@ from .models import (
     ModifyingSaveData, NullBooleanData, O2OData, PositiveIntegerData,
     PositiveIntegerPKData, PositiveSmallIntegerData,
     PositiveSmallIntegerPKData, SlugData, SlugPKData, SmallData, SmallPKData,
-    Tag, TextData, TimeData, UniqueAnchor, UUIDData,
+    Tag, TextData, TimeData, UniqueAnchor, UUIDData, UUIDDefaultData,
 )
 from .tests import register_tests
 
@@ -349,6 +349,7 @@ The end."""),
     # (pk_obj, 790, XMLPKData, "<foo></foo>"),
     (pk_obj, 791, UUIDData, uuid_obj),
     (fk_obj, 792, FKToUUID, uuid_obj),
+    (pk_obj, 793, UUIDDefaultData, uuid_obj),
 
     (data_obj, 800, AutoNowDateTimeData, datetime.datetime(2006, 6, 16, 10, 42, 37)),
     (data_obj, 810, ModifyingSaveData, 42),