Fixed #470 -- Added support for database defaults on fields.

Special thanks to Hannes Ljungberg for finding multiple implementation
gaps.

Thanks also to Simon Charette, Adam Johnson, and Mariusz Felisiak for
reviews.

5 files changed, 80 insertions, 5 deletions

diff --git a/docs/ref/checks.txt b/docs/ref/checks.txt
index 9e350a3ff3..df0adbef63 100644
--- a/docs/ref/checks.txt
+++ b/docs/ref/checks.txt
@@ -175,6 +175,9 @@ Model fields
   ``choices`` (``<count>`` characters).
 * **fields.E010**: ``<field>`` default should be a callable instead of an
   instance so that it's not shared between all field instances.
+* **fields.E011**: ``<database>`` does not support default database values with
+  expressions (``db_default``).
+* **fields.E012**: ``<expression>`` cannot be used in ``db_default``.
 * **fields.E100**: ``AutoField``\s must set primary_key=True.
 * **fields.E110**: ``BooleanField``\s do not accept null values. *This check
   appeared before support for null values was added in Django 2.1.*
diff --git a/docs/ref/models/expressions.txt b/docs/ref/models/expressions.txt
index 52a1022771..50560dfa9b 100644
--- a/docs/ref/models/expressions.txt
+++ b/docs/ref/models/expressions.txt
@@ -996,6 +996,13 @@ calling the appropriate methods on the wrapped expression.
 
 .. class:: Expression
 
+    .. attribute:: allowed_default
+
+        .. versionadded:: 5.0
+
+        Tells Django that this expression can be used in
+        :attr:`Field.db_default`. Defaults to ``False``.
+
     .. attribute:: contains_aggregate
 
         Tells Django that this expression contains an aggregate and that a
diff --git a/docs/ref/models/fields.txt b/docs/ref/models/fields.txt
index 27b87c1f53..344cc45280 100644
--- a/docs/ref/models/fields.txt
+++ b/docs/ref/models/fields.txt
@@ -351,6 +351,38 @@ looking at your Django code. For example::
         db_comment="Date and time when the article was published",
     )
 
+``db_default``
+--------------
+
+.. versionadded:: 5.0
+
+.. attribute:: Field.db_default
+
+The database-computed default value for this field. This can be a literal value
+or a database function, such as :class:`~django.db.models.functions.Now`::
+
+    created = models.DateTimeField(db_default=Now())
+
+More complex expressions can be used, as long as they are made from literals
+and database functions::
+
+    month_due = models.DateField(
+        db_default=TruncMonth(
+            Now() + timedelta(days=90),
+            output_field=models.DateField(),
+        )
+    )
+
+Database defaults cannot reference other fields or models. For example, this is
+invalid::
+
+    end = models.IntegerField(db_default=F("start") + 50)
+
+If both ``db_default`` and :attr:`Field.default` are set, ``default`` will take
+precedence when creating instances in Python code. ``db_default`` will still be
+set at the database level and will be used when inserting rows outside of the
+ORM or when adding a new field in a migration.
+
 ``db_index``
 ------------
 
@@ -408,6 +440,9 @@ 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``.
 
+The default value can also be set at the database level with
+:attr:`Field.db_default`.
+
 ``editable``
 ------------
 
diff --git a/docs/ref/models/instances.txt b/docs/ref/models/instances.txt
index d03b05577c..346ae55130 100644
--- a/docs/ref/models/instances.txt
+++ b/docs/ref/models/instances.txt
@@ -541,7 +541,8 @@ You may have noticed Django database objects use the same ``save()`` method
 for creating and changing objects. Django abstracts the need to use ``INSERT``
 or ``UPDATE`` SQL statements. Specifically, when you call ``save()`` and the
 object's primary key attribute does **not** define a
-:attr:`~django.db.models.Field.default`, Django follows this algorithm:
+:attr:`~django.db.models.Field.default` or
+:attr:`~django.db.models.Field.db_default`, Django follows this algorithm:
 
 * If the object's primary key attribute is set to a value that evaluates to
   ``True`` (i.e., a value other than ``None`` or the empty string), Django
@@ -551,9 +552,10 @@ object's primary key attribute does **not** define a
   exist in the database), Django executes an ``INSERT``.
 
 If the object's primary key attribute defines a
-:attr:`~django.db.models.Field.default` then Django executes an ``UPDATE`` if
-it is an existing model instance and primary key is set to a value that exists
-in the database. Otherwise, Django executes an ``INSERT``.
+:attr:`~django.db.models.Field.default` or
+:attr:`~django.db.models.Field.db_default` then Django executes an ``UPDATE``
+if it is an existing model instance and primary key is set to a value that
+exists in the database. Otherwise, Django executes an ``INSERT``.
 
 The one gotcha here is that you should be careful not to specify a primary-key
 value explicitly when saving new objects, if you cannot guarantee the
@@ -570,6 +572,10 @@ which returns ``NULL``. In such cases it is possible to revert to the old
 algorithm by setting the :attr:`~django.db.models.Options.select_on_save`
 option to ``True``.
 
+.. versionchanged:: 5.0
+
+    The ``Field.db_default`` parameter was added.
+
 .. _ref-models-force-insert:
 
 Forcing an INSERT or UPDATE
diff --git a/docs/releases/5.0.txt b/docs/releases/5.0.txt
index f23f39b014..d40cd6a4f0 100644
--- a/docs/releases/5.0.txt
+++ b/docs/releases/5.0.txt
@@ -108,6 +108,21 @@ Can now be simplified to:
 per-project, per-field, or per-request basis. See
 :ref:`reusable-field-group-templates`.
 
+Database-computed default values
+--------------------------------
+
+The new :attr:`Field.db_default <django.db.models.Field.db_default>` parameter
+sets a database-computed default value. For example::
+
+    from django.db import models
+    from django.db.models.functions import Now, Pi
+
+
+    class MyModel(models.Model):
+        age = models.IntegerField(db_default=18)
+        created = models.DateTimeField(db_default=Now())
+        circumference = models.FloatField(db_default=2 * Pi())
+
 Minor features
 --------------
 
@@ -355,7 +370,16 @@ Database backend API
 This section describes changes that may be needed in third-party database
 backends.
 
-* ...
+* ``DatabaseFeatures.supports_expression_defaults`` should be set to ``False``
+  if the database doesn't support using database functions as defaults.
+
+* ``DatabaseFeatures.supports_default_keyword_in_insert`` should be set to
+  ``False`` if the database doesn't support the ``DEFAULT`` keyword in
+  ``INSERT`` queries.
+
+* ``DatabaseFeatures.supports_default_keyword_in_bulk insert`` should be set to
+  ``False`` if the database doesn't support the ``DEFAULT`` keyword in bulk
+  ``INSERT`` queries.
 
 Using ``create_defaults__exact`` may now be required with ``QuerySet.update_or_create()``
 -----------------------------------------------------------------------------------------