Fixed #24932 -- Added Cast database function.

Thanks Ian Foote for the initial patch.

django/db/models/functions.py

@@ -4,6 +4,39 @@ Classes that represent database functions.
 from django.db.models import Func, Transform, Value, fields
 
 
+class Cast(Func):
+    """
+    Coerce an expression to a new field type.
+    """
+    function = 'CAST'
+    template = '%(function)s(%(expressions)s AS %(db_type)s)'
+
+    mysql_types = {
+        fields.CharField: 'char',
+        fields.IntegerField: 'signed integer',
+        fields.FloatField: 'signed',
+    }
+
+    def __init__(self, expression, output_field):
+        super(Cast, self).__init__(expression, output_field=output_field)
+
+    def as_sql(self, compiler, connection, **extra_context):
+        if 'db_type' not in extra_context:
+            extra_context['db_type'] = self._output_field.db_type(connection)
+        return super(Cast, self).as_sql(compiler, connection, **extra_context)
+
+    def as_mysql(self, compiler, connection):
+        extra_context = {}
+        output_field_class = type(self._output_field)
+        if output_field_class in self.mysql_types:
+            extra_context['db_type'] = self.mysql_types[output_field_class]
+        return self.as_sql(compiler, connection, **extra_context)
+
+    def as_postgresql(self, compiler, connection):
+        # CAST would be valid too, but the :: shortcut syntax is more readable.
+        return self.as_sql(compiler, connection, template='%(expressions)s::%(db_type)s')
+
+
 class Coalesce(Func):
     """
     Chooses, from left to right, the first non-null expression and returns it.

docs/ref/models/database-functions.txt

@@ -23,6 +23,24 @@ We don't usually recommend allowing ``null=True`` for ``CharField`` since this
 allows the field to have two "empty values", but it's important for the
 ``Coalesce`` example below.
 
+``Cast``
+========
+
+.. class:: Cast(expression, output_field)
+
+.. versionadded:: 1.10
+
+Forces the result type of ``expression`` to be the one from ``output_field``.
+
+Usage example::
+
+    >>> from django.db.models import FloatField
+    >>> from django.db.models.functions import Cast
+    >>> Value.objects.create(integer=4)
+    >>> value = Value.objects.annotate(as_float=Cast('integer', FloatField)).get()
+    >>> print(value.as_float)
+    4.0
+
 ``Coalesce``
 ============
 

docs/releases/1.10.txt

@@ -349,6 +349,8 @@ Models
 * :meth:`QuerySet.bulk_create() <django.db.models.query.QuerySet.bulk_create>`
   sets the primary key on objects when using PostgreSQL.
 
+* Added the :class:`~django.db.models.functions.Cast` database function.
+
 Requests and Responses
 ~~~~~~~~~~~~~~~~~~~~~~
 

tests/db_functions/test_cast.py

+from django.db import models
+from django.db.models.expressions import Value
+from django.db.models.functions import Cast
+from django.test import TestCase
+
+from .models import Author
+
+
+class CastTests(TestCase):
+    @classmethod
+    def setUpTestData(self):
+        Author.objects.create(name='Bob', age=1)
+
+    def test_cast_from_value(self):
+        numbers = Author.objects.annotate(cast_integer=Cast(Value('0'), models.IntegerField()))
+        self.assertEqual(numbers.get().cast_integer, 0)
+
+    def test_cast_from_field(self):
+        numbers = Author.objects.annotate(cast_string=Cast('age', models.CharField(max_length=255)),)
+        self.assertEqual(numbers.get().cast_string, '1')
+
+    def test_cast_from_python(self):
+        numbers = Author.objects.annotate(cast_float=Cast(0, models.FloatField()))
+        self.assertEqual(numbers.get().cast_float, 0.0)