Added Model.has_db_default() to encapsulate NOT_PROVIDED checks.

This avoids many awkward checks against NOT_PROVIDED and provides symmetry
with Field.has_default() which is also the reason why it wasn't made a
property.

django/db/backends/base/schema.py

@@ -13,7 +13,7 @@ from django.db.backends.ddl_references import (
     Table,
 )
 from django.db.backends.utils import names_digest, split_identifier, truncate_name
-from django.db.models import NOT_PROVIDED, Deferrable, Index
+from django.db.models import Deferrable, Index
 from django.db.models.fields.composite import CompositePrimaryKey
 from django.db.models.sql import Query
 from django.db.transaction import TransactionManagementError, atomic
@@ -318,7 +318,7 @@ class BaseDatabaseSchemaEditor:
         # Work out nullability.
         null = field.null
         # Add database default.
-        if field.db_default is not NOT_PROVIDED:
+        if field.has_db_default():
             default_sql, default_params = self.db_default_sql(field)
             yield f"DEFAULT {default_sql}"
             params.extend(default_params)
@@ -775,7 +775,7 @@ class BaseDatabaseSchemaEditor:
         self.execute(sql, params or None)
         # Drop the default if we need to
         if (
-            field.db_default is NOT_PROVIDED
+            not field.has_db_default()
             and not self.skip_default_on_alter(field)
             and self.effective_default(field) is not None
         ):
@@ -1108,15 +1108,15 @@ class BaseDatabaseSchemaEditor:
             actions.append(fragment)
             post_actions.extend(other_actions)
 
-        if new_field.db_default is not NOT_PROVIDED:
+        if new_field.has_db_default():
             if (
-                old_field.db_default is NOT_PROVIDED
+                not old_field.has_db_default()
                 or new_field.db_default != old_field.db_default
             ):
                 actions.append(
                     self._alter_column_database_default_sql(model, old_field, new_field)
                 )
-        elif old_field.db_default is not NOT_PROVIDED:
+        elif old_field.has_db_default():
             actions.append(
                 self._alter_column_database_default_sql(
                     model, old_field, new_field, drop=True
@@ -1130,11 +1130,7 @@ class BaseDatabaseSchemaEditor:
         #  4. Drop the default again.
         # Default change?
         needs_database_default = False
-        if (
-            old_field.null
-            and not new_field.null
-            and new_field.db_default is NOT_PROVIDED
-        ):
+        if old_field.null and not new_field.null and not new_field.has_db_default():
             old_default = self.effective_default(old_field)
             new_default = self.effective_default(new_field)
             if (
@@ -1153,7 +1149,7 @@ class BaseDatabaseSchemaEditor:
                 null_actions.append(fragment)
         # Only if we have a default and there is a change from NULL to NOT NULL
         four_way_default_alteration = (
-            new_field.has_default() or new_field.db_default is not NOT_PROVIDED
+            new_field.has_default() or new_field.has_db_default()
         ) and (old_field.null and not new_field.null)
         if actions or null_actions:
             if not four_way_default_alteration:
@@ -1175,7 +1171,7 @@ class BaseDatabaseSchemaEditor:
                     params,
                 )
             if four_way_default_alteration:
-                if new_field.db_default is NOT_PROVIDED:
+                if not new_field.has_db_default():
                     default_sql = "%s"
                     params = [new_default]
                 else:

django/db/backends/mysql/schema.py

@@ -222,7 +222,7 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         Keep the NULL and DEFAULT properties of the old field. If it has
         changed, it will be handled separately.
         """
-        if field.db_default is not NOT_PROVIDED:
+        if field.has_db_default():
             default_sql, params = self.db_default_sql(field)
             default_sql %= tuple(self.quote_value(p) for p in params)
             new_type += f" DEFAULT {default_sql}"
@@ -266,7 +266,7 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         return f" COMMENT {comment_sql}"
 
     def _alter_column_null_sql(self, model, old_field, new_field):
-        if new_field.db_default is NOT_PROVIDED:
+        if not new_field.has_db_default():
             return super()._alter_column_null_sql(model, old_field, new_field)
 
         new_db_params = new_field.db_parameters(connection=self.connection)

django/db/backends/sqlite3/schema.py

@@ -6,7 +6,7 @@ from django.db import NotSupportedError
 from django.db.backends.base.schema import BaseDatabaseSchemaEditor
 from django.db.backends.ddl_references import Statement
 from django.db.backends.utils import strip_quotes
-from django.db.models import NOT_PROVIDED, CompositePrimaryKey, UniqueConstraint
+from django.db.models import CompositePrimaryKey, UniqueConstraint
 
 
 class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
@@ -144,7 +144,7 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
             body[create_field.name] = create_field
             # Choose a default and insert it into the copy map
             if (
-                create_field.db_default is NOT_PROVIDED
+                not create_field.has_db_default()
                 and not (create_field.many_to_many or create_field.generated)
                 and create_field.concrete
             ):
@@ -161,7 +161,7 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
             if new_field.generated:
                 continue
             if old_field.null and not new_field.null:
-                if new_field.db_default is NOT_PROVIDED:
+                if not new_field.has_db_default():
                     default = self.prepare_default(self.effective_default(new_field))
                 else:
                     default, _ = self.db_default_sql(new_field)
@@ -321,10 +321,7 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
             or self.effective_default(field) is not None
             # Fields with non-constant defaults cannot by handled by ALTER
             # TABLE ADD COLUMN statement.
-            or (
-                field.db_default is not NOT_PROVIDED
-                and not isinstance(field.db_default, Value)
-            )
+            or (field.has_db_default() and not isinstance(field.db_default, Value))
         ):
             self._remake_table(model, create_field=field)
         else:

django/db/migrations/autodetector.py

@@ -1135,7 +1135,7 @@ class MigrationAutodetector:
         preserve_default = (
             field.null
             or field.has_default()
-            or field.db_default is not models.NOT_PROVIDED
+            or field.has_db_default()
             or field.many_to_many
             or (field.blank and field.empty_strings_allowed)
             or (isinstance(field, time_fields) and field.auto_now)
@@ -1150,11 +1150,7 @@ class MigrationAutodetector:
                 field.default = self.questioner.ask_not_null_addition(
                     field_name, model_name
                 )
-        if (
-            field.unique
-            and field.default is not models.NOT_PROVIDED
-            and callable(field.default)
-        ):
+        if field.unique and field.has_default() and callable(field.default):
             self.questioner.ask_unique_callable_default_addition(field_name, model_name)
         self.add_operation(
             app_label,
@@ -1296,7 +1292,7 @@ class MigrationAutodetector:
                         old_field.null
                         and not new_field.null
                         and not new_field.has_default()
-                        and new_field.db_default is models.NOT_PROVIDED
+                        and not new_field.has_db_default()
                         and not new_field.many_to_many
                     ):
                         field = new_field.clone()

django/db/models/base.py

@@ -1117,10 +1117,7 @@ class Model(AltersData, metaclass=ModelBase):
             and not force_insert
             and not force_update
             and self._state.adding
-            and (
-                (meta.pk.default and meta.pk.default is not NOT_PROVIDED)
-                or (meta.pk.db_default and meta.pk.db_default is not NOT_PROVIDED)
-            )
+            and (meta.pk.has_default() or meta.pk.has_db_default())
         ):
             force_insert = True
         # If possible, try an UPDATE. If that doesn't update anything, do an INSERT.

django/db/models/fields/__init__.py

@@ -391,7 +391,7 @@ class Field(RegisterLookupMixin):
         from django.db.models.expressions import Value
 
         if (
-            self.db_default is NOT_PROVIDED
+            not self.has_db_default()
             or (
                 isinstance(self.db_default, Value)
                 or not hasattr(self.db_default, "resolve_expression")
@@ -933,8 +933,7 @@ class Field(RegisterLookupMixin):
     def db_returning(self):
         """Private API intended only to be used by Django itself."""
         return (
-            self.db_default is not NOT_PROVIDED
-            and connection.features.can_return_columns_from_insert
+            self.has_db_default() and connection.features.can_return_columns_from_insert
         )
 
     def set_attributes_from_name(self, name):
@@ -1016,6 +1015,10 @@ class Field(RegisterLookupMixin):
         """Return a boolean of whether this field has a default value."""
         return self.default is not NOT_PROVIDED
 
+    def has_db_default(self):
+        """Return a boolean of whether this field has a db_default value."""
+        return self.db_default is not NOT_PROVIDED
+
     def get_default(self):
         """Return the default value for this field."""
         return self._get_default()
@@ -1027,7 +1030,7 @@ class Field(RegisterLookupMixin):
                 return self.default
             return lambda: self.default
 
-        if self.db_default is not NOT_PROVIDED:
+        if self.has_db_default():
             from django.db.models.expressions import DatabaseDefault
 
             return lambda: DatabaseDefault(
@@ -1045,9 +1048,7 @@ class Field(RegisterLookupMixin):
     @cached_property
     def _db_default_expression(self):
         db_default = self.db_default
-        if db_default is not NOT_PROVIDED and not hasattr(
-            db_default, "resolve_expression"
-        ):
+        if self.has_db_default() and not hasattr(db_default, "resolve_expression"):
             from django.db.models.expressions import Value
 
             db_default = Value(db_default, self)