Resolved #30969 -- Added support for query expressions as default values

django/db/models/fields/__init__.py

@@ -738,7 +738,8 @@ def db_returning(self):
         Private API intended only to be used by Django itself. Currently only
         the PostgreSQL backend supports returning multiple fields on a model.
         """
-        return False
+        from django.db.models.expressions import BaseExpression
+        return self.has_default() and isinstance(self.default, BaseExpression)
 
     def set_attributes_from_name(self, name):
         self.name = self.name or name

docs/ref/models/fields.txt

@@ -364,6 +364,19 @@ The default value is used when new model instances are created and a value
 isn't provided for the field. When the field is a primary key, the default is
 also used when the field is set to ``None``.
 
+.. versionadded:: 3.1
+
+The Oracle and PostgreSQL database backends support
+:doc:`Query Expressions </ref/models/expressions>` as ``default`` values::
+
+    from django.db import models
+    from django.db.models.functions import ExtractYear, Now, Pi
+
+    class CreatedModel(models.Model)
+        created = models.DateTimeField(editable=False, default=Now())
+        pi = models.FloatField(default=Pi())
+        year = models.PositiveSmallIntegerField(default=ExtractYear(Now()))
+
 ``editable``
 ------------
 

docs/releases/3.1.txt

@@ -189,6 +189,9 @@ Pagination
 
 * :class:`~django.core.paginator.Paginator` can now be iterated over to yield
   its pages.
+* The Oracle and PostgreSQL backends support
+  :doc:`Query Expressions </ref/models/expressions>` as default values for model
+  fields.
 
 Requests and Responses
 ~~~~~~~~~~~~~~~~~~~~~~

tests/queries/models.py

@@ -4,7 +4,7 @@
 import threading
 
 from django.db import models
-from django.db.models.functions import Now
+from django.db.models.functions import ExtractYear, Now, Pi
 
 
 class DumbCategory(models.Model):
@@ -733,17 +733,12 @@ class CustomDbColumn(models.Model):
     ip_address = models.GenericIPAddressField(null=True)
 
 
-class CreatedField(models.DateTimeField):
-    db_returning = True
-
-    def __init__(self, *args, **kwargs):
-        kwargs.setdefault('default', Now)
-        super().__init__(*args, **kwargs)
-
-
 class ReturningModel(models.Model):
-    created = CreatedField(editable=False)
+    created = models.DateTimeField(editable=False, default=Now())
+    created_date = models.DateTimeField(default=Now())
+    year = models.PositiveSmallIntegerField(default=ExtractYear(Now()))
+    pi = models.FloatField(default=Pi())
 
 
 class NonIntegerPKReturningModel(models.Model):
-    created = CreatedField(editable=False, primary_key=True)
+    created = models.DateTimeField(editable=False, primary_key=True, default=Now())

tests/queries/test_db_returning.py

@@ -49,3 +49,12 @@ def test_bulk_insert(self):
             with self.subTest(obj=obj):
                 self.assertTrue(obj.pk)
                 self.assertIsInstance(obj.created, datetime.datetime)
+
+
+@skipUnlessDBFeature('can_return_columns_from_insert')
+class DatabaseDefaultsTests(TestCase):
+    def test_now(self):
+        obj = ReturningModel.objects.create()
+        self.assertIsInstance(obj.created, datetime.datetime)
+        self.assertIsInstance(obj.year, int)
+        self.assertEquals(obj.pi, 3.14159265358979)