Fixed #25160 -- Moved unsaved model instance data loss check to Model.save()

This mostly reverts 5643a3b51b and
81e1a35c36.

Thanks Carl Meyer for review.

tests/model_fields/test_uuid.py

@@ -2,8 +2,10 @@ import json
 import uuid
 
 from django.core import exceptions, serializers
-from django.db import models
-from django.test import SimpleTestCase, TestCase
+from django.db import IntegrityError, models
+from django.test import (
+    SimpleTestCase, TestCase, TransactionTestCase, skipUnlessDBFeature,
+)
 
 from .models import (
     NullableUUIDModel, PrimaryKeyUUIDModel, RelatedToUUIDModel, UUIDGrandchild,
@@ -158,3 +160,14 @@ class TestAsPrimaryKey(TestCase):
     def test_two_level_foreign_keys(self):
         # exercises ForeignKey.get_db_prep_value()
         UUIDGrandchild().save()
+
+
+class TestAsPrimaryKeyTransactionTests(TransactionTestCase):
+    # Need a TransactionTestCase to avoid deferring FK constraint checking.
+    available_apps = ['model_fields']
+
+    @skipUnlessDBFeature('supports_foreign_keys')
+    def test_unsaved_fk(self):
+        u1 = PrimaryKeyUUIDModel()
+        with self.assertRaises(IntegrityError):
+            RelatedToUUIDModel.objects.create(uuid_fk=u1)