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.