Replace auto_now + default=Now/GenRandomUUID with type-scoped kwargs

DB-default behavior was overloaded onto `default=` via a generic
`DatabaseDefaultExpression` marker. Users wrote `default=Now()` on
DateTimeField and `default=GenRandomUUID()` on UUIDField, and nothing
stopped `default=Now()` from being typed on an IntegerField. This
change narrows the surface to each field type's actual use case:

- DateTimeField(create_now=True) — DB STATEMENT_TIMESTAMP() on INSERT
- DateTimeField(update_now=True) — Python pre_save, fires on every save
- UUIDField(generate=True) — DB gen_random_uuid() on INSERT

`default=` is removed entirely from DateTimeField and UUIDField.
DateField/TimeField keep `default=` and drop the rarely-useful `auto_now`.
`Now()` and `GenRandomUUID()` stay exported for use in annotations and
filters; they're no longer valid as `default=` values.

Internal cleanup:
- `DatabaseDefaultExpression` marker class deleted; `Now`/`GenRandomUUID`
  are plain `Func` subclasses.
- `_db_default_expression` moved off `self.default` onto a per-field
  `cached_property`; expression construction is derived from the boolean
  kwarg.
- `has_db_default()` routes through a new `get_db_default_expression()`
  method on Field; `db_returning` is now a property that delegates to it.
- `auto_fills_on_save` property on Field lets full_clean and the
  autodetector skip validation/backfill for pre_save-filled fields.
- `has_any_default()` consolidates the three-way predicate used by
  autodetector guards and the schema editor's four-way backfill gate.
- DateTimeField no longer inherits from DateField; temporal lookups
  (ExtractYear/Month/Day/etc.) are registered on DateTimeField
  explicitly, and TruncBase/CombinedExpression isinstance checks now
  include DateTimeField.
- UUIDField inherits from ColumnField (not DefaultableField).

All shipped model fields and migrations are converted to the new API.
The plainmigrations recorder uses `update_now=True` (not `create_now=True`)
because ensure_schema() skips tables that already exist — existing
installations wouldn't gain the DB default.

Author: davegaeddert

README.md

@@ -31,7 +31,6 @@ Models are Postgres-only:
 # app/users/models.py
 from plain import postgres
 from plain.postgres import types
-from plain.postgres.functions import Now
 from plain.passwords.models import PasswordField
 
 @postgres.register_model
@@ -40,7 +39,7 @@ class User(postgres.Model):
     password: str = PasswordField()
     display_name: str = types.TextField(max_length=100)
     is_admin: bool = types.BooleanField(default=False)
-    created_at: datetime = types.DateTimeField(default=Now())
+    created_at: datetime = types.DateTimeField(create_now=True)
 
     query: postgres.QuerySet[User] = postgres.QuerySet()
 

example/app/notes/migrations/0001_initial.py

@@ -3,7 +3,6 @@
 import plain.postgres.deletion
 from plain import postgres
 from plain.postgres import migrations
-from plain.postgres.functions import Now
 
 
 class Migration(migrations.Migration):
@@ -19,9 +18,9 @@ class Migration(migrations.Migration):
             fields=[
                 ("id", postgres.PrimaryKeyField()),
                 ("body", postgres.TextField(default="", required=False)),
-                ("created_at", postgres.DateTimeField(default=Now())),
+                ("created_at", postgres.DateTimeField(create_now=True)),
                 ("title", postgres.TextField(max_length=200)),
-                ("updated_at", postgres.DateTimeField(auto_now=True)),
+                ("updated_at", postgres.DateTimeField(update_now=True)),
                 (
                     "author",
                     postgres.ForeignKeyField(

example/app/notes/models.py

@@ -5,7 +5,6 @@
 from app.users.models import User
 from plain import postgres
 from plain.postgres import types
-from plain.postgres.functions import Now
 from plain.urls import reverse
 
 
@@ -18,8 +17,8 @@ class Note(postgres.Model):
     )
     title: str = types.TextField(max_length=200)
     body: str = types.TextField(default="", required=False)
-    created_at: datetime.datetime = types.DateTimeField(default=Now())
-    updated_at: datetime.datetime = types.DateTimeField(auto_now=True)
+    created_at: datetime.datetime = types.DateTimeField(create_now=True)
+    updated_at: datetime.datetime = types.DateTimeField(update_now=True)
 
     query: postgres.QuerySet[Note] = postgres.QuerySet()
 

example/app/users/migrations/0002_user_created_at.py

@@ -2,7 +2,6 @@
 
 from plain import postgres
 from plain.postgres import migrations
-from plain.postgres.functions import Now
 
 
 class Migration(migrations.Migration):
@@ -14,6 +13,6 @@ class Migration(migrations.Migration):
         migrations.AddField(
             model_name="user",
             name="created_at",
-            field=postgres.DateTimeField(default=Now()),
+            field=postgres.DateTimeField(create_now=True),
         ),
     ]

example/app/users/models.py

@@ -5,14 +5,13 @@
 from plain import postgres
 from plain.passwords.types import PasswordField
 from plain.postgres import types
-from plain.postgres.functions import Now
 
 
 @postgres.register_model
 class User(postgres.Model):
     email: str = types.EmailField()
     password: str = PasswordField()
     is_admin: bool = types.BooleanField(default=False)
-    created_at: datetime.datetime = types.DateTimeField(default=Now())
+    created_at: datetime.datetime = types.DateTimeField(create_now=True)
 
     query: postgres.QuerySet[User] = postgres.QuerySet()

plain-admin/plain/admin/migrations/0001_initial.py

@@ -3,7 +3,6 @@
 import plain.postgres.deletion
 from plain import postgres
 from plain.postgres import migrations
-from plain.postgres.functions import Now
 
 
 class Migration(migrations.Migration):
@@ -18,7 +17,7 @@ class Migration(migrations.Migration):
             name="PinnedNavItem",
             fields=[
                 ("id", postgres.PrimaryKeyField()),
-                ("created_at", postgres.DateTimeField(default=Now())),
+                ("created_at", postgres.DateTimeField(create_now=True)),
                 ("order", postgres.SmallIntegerField(default=0)),
                 ("view_slug", postgres.TextField(max_length=255)),
                 (

plain-admin/plain/admin/models.py

@@ -4,7 +4,6 @@
 
 from plain import postgres
 from plain.postgres import types
-from plain.postgres.functions import Now
 
 
 @postgres.register_model
@@ -17,7 +16,7 @@ class PinnedNavItem(postgres.Model):
     )
     view_slug: str = types.TextField(max_length=255)
     order: int = types.SmallIntegerField(default=0)
-    created_at: datetime = types.DateTimeField(default=Now())
+    created_at: datetime = types.DateTimeField(create_now=True)
 
     query: postgres.QuerySet[PinnedNavItem] = postgres.QuerySet()
 

plain-api/plain/api/migrations/0001_initial.py

@@ -1,11 +1,9 @@
 # Generated by Plain 0.52.2 on 2025-07-08 01:17
 
-import uuid
 
 import plain.api.models
 from plain import postgres
 from plain.postgres import migrations
-from plain.postgres.functions import Now
 
 
 class Migration(migrations.Migration):
@@ -18,9 +16,9 @@ class Migration(migrations.Migration):
             name="APIKey",
             fields=[
                 ("id", postgres.PrimaryKeyField()),
-                ("uuid", postgres.UUIDField(default=uuid.uuid4)),
-                ("created_at", postgres.DateTimeField(default=Now())),
-                ("updated_at", postgres.DateTimeField(auto_now=True)),
+                ("uuid", postgres.UUIDField(generate=True)),
+                ("created_at", postgres.DateTimeField(create_now=True)),
+                ("updated_at", postgres.DateTimeField(update_now=True)),
                 ("expires_at", postgres.DateTimeField(allow_null=True, required=False)),
                 (
                     "last_used_at",

plain-api/plain/api/models.py

@@ -2,13 +2,11 @@
 
 import binascii
 import os
-import uuid
 from datetime import datetime
 from uuid import UUID
 
 from plain import postgres
 from plain.postgres import types
-from plain.postgres.functions import Now
 
 __all__ = ["APIKey"]
 
@@ -19,9 +17,9 @@ def generate_token() -> str:
 
 @postgres.register_model
 class APIKey(postgres.Model):
-    uuid: UUID = types.UUIDField(default=uuid.uuid4)
-    created_at: datetime = types.DateTimeField(default=Now())
-    updated_at: datetime = types.DateTimeField(auto_now=True)
+    uuid: UUID = types.UUIDField(generate=True)
+    created_at: datetime = types.DateTimeField(create_now=True)
+    updated_at: datetime = types.DateTimeField(update_now=True)
     expires_at: datetime | None = types.DateTimeField(required=False, allow_null=True)
     last_used_at: datetime | None = types.DateTimeField(required=False, allow_null=True)
 

plain-auth/plain/auth/README.md

@@ -76,7 +76,6 @@ from datetime import datetime
 
 from plain import postgres
 from plain.postgres import types
-from plain.postgres.functions import Now
 from plain.passwords.models import PasswordField
 
 
@@ -85,7 +84,7 @@ class User(postgres.Model):
     email: str = types.EmailField()
     password = PasswordField()
     is_admin: bool = types.BooleanField(default=False)
-    created_at: datetime = types.DateTimeField(default=Now())
+    created_at: datetime = types.DateTimeField(create_now=True)
 
     def __str__(self):
         return self.email