Allowed database backends to specify data types for Cast().

A small refactor ahead of refs #28371.

django/db/backends/base/operations.py

@@ -32,6 +32,10 @@ class BaseDatabaseOperations:
         'intersection': 'INTERSECT',
         'difference': 'EXCEPT',
     }
+    # Mapping of Field.get_internal_type() (typically the model field's class
+    # name) to the data type to use for the Cast() function, if different from
+    # DatabaseWrapper.data_types.
+    cast_data_types = {}
 
     def __init__(self, connection):
         self.connection = connection

django/db/backends/mysql/operations.py

@@ -15,6 +15,15 @@ class DatabaseOperations(BaseDatabaseOperations):
         PositiveSmallIntegerField=(0, 65535),
         PositiveIntegerField=(0, 4294967295),
     )
+    cast_data_types = {
+        'CharField': 'char(%(max_length)s)',
+        'IntegerField': 'signed integer',
+        'BigIntegerField': 'signed integer',
+        'SmallIntegerField': 'signed integer',
+        'FloatField': 'signed',
+        'PositiveIntegerField': 'unsigned integer',
+        'PositiveSmallIntegerField': 'unsigned integer',
+    }
 
     def date_extract_sql(self, lookup_type, field_name):
         # http://dev.mysql.com/doc/mysql/en/date-and-time-functions.html

django/db/models/fields/__init__.py

@@ -656,6 +656,13 @@ class Field(RegisterLookupMixin):
         """
         return self.db_type(connection)
 
+    def cast_db_type(self, connection):
+        """Return the data type to use in the Cast() function."""
+        db_type = connection.ops.cast_data_types.get(self.get_internal_type())
+        if db_type:
+            return db_type % self.db_type_parameters(connection)
+        return self.db_type(connection)
+
     def db_parameters(self, connection):
         """
         Extension of db_type(), providing a range of different return values

django/db/models/functions/base.py

@@ -9,32 +9,13 @@ class Cast(Func):
     function = 'CAST'
     template = '%(function)s(%(expressions)s AS %(db_type)s)'
 
-    mysql_types = {
-        fields.CharField: 'char(%(max_length)s)',
-        fields.IntegerField: 'signed integer',
-        fields.BigIntegerField: 'signed integer',
-        fields.SmallIntegerField: 'signed integer',
-        fields.FloatField: 'signed',
-        fields.PositiveIntegerField: 'unsigned integer',
-        fields.PositiveSmallIntegerField: 'unsigned integer',
-    }
-
     def __init__(self, expression, output_field):
         super().__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.cast_db_type(connection)
-            extra_context['db_type'] = self.output_field.db_type(connection)
         return super().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:
-            data = self.output_field.db_type_parameters(connection)
-            extra_context['db_type'] = self.mysql_types[output_field_class] % data
-        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')