Fixed #28371 -- Fixed Cast() with CharField if the max_length argument isn't provided.

Thanks Tim Graham for the review.

django/db/backends/base/operations.py

@@ -36,6 +36,8 @@ class BaseDatabaseOperations:
     # name) to the data type to use for the Cast() function, if different from
     # DatabaseWrapper.data_types.
     cast_data_types = {}
+    # CharField data type if the max_length argument isn't provided.
+    cast_char_field_without_max_length = None
 
     def __init__(self, connection):
         self.connection = connection

django/db/backends/mysql/operations.py

@@ -24,6 +24,7 @@ class DatabaseOperations(BaseDatabaseOperations):
         'PositiveIntegerField': 'unsigned integer',
         'PositiveSmallIntegerField': 'unsigned integer',
     }
+    cast_char_field_without_max_length = 'char'
 
     def date_extract_sql(self, lookup_type, field_name):
         # http://dev.mysql.com/doc/mysql/en/date-and-time-functions.html

django/db/backends/oracle/operations.py

@@ -49,6 +49,9 @@ BEGIN
 END;
 /"""
 
+    # Oracle doesn't support string without precision; use the max string size.
+    cast_char_field_without_max_length = 'NVARCHAR2(2000)'
+
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
         self.set_operators['difference'] = 'MINUS'

django/db/backends/postgresql/operations.py

@@ -5,6 +5,8 @@ from django.db.backends.base.operations import BaseDatabaseOperations
 
 
 class DatabaseOperations(BaseDatabaseOperations):
+    cast_char_field_without_max_length = 'varchar'
+
     def unification_cast_sql(self, output_field):
         internal_type = output_field.get_internal_type()
         if internal_type in ("GenericIPAddressField", "IPAddressField", "TimeField", "UUIDField"):

django/db/backends/sqlite3/operations.py

@@ -14,6 +14,8 @@ from django.utils.duration import duration_string
 
 
 class DatabaseOperations(BaseDatabaseOperations):
+    cast_char_field_without_max_length = 'text'
+
     def bulk_batch_size(self, fields, objs):
         """
         SQLite has a compile-time default (SQLITE_LIMIT_VARIABLE_NUMBER) of

django/db/models/fields/__init__.py

@@ -1066,6 +1066,11 @@ class CharField(Field):
         else:
             return []
 
+    def cast_db_type(self, connection):
+        if self.max_length is None:
+            return connection.ops.cast_char_field_without_max_length
+        return super().cast_db_type(connection)
+
     def get_internal_type(self):
         return "CharField"
 

docs/releases/2.0.txt

@@ -348,6 +348,12 @@ backends.
   requires that the arguments to ``OF`` be columns rather than tables, set
   ``DatabaseFeatures.select_for_update_of_column = True``.
 
+* Third-party database backends should add a
+  ``DatabaseOperations.cast_char_field_without_max_length`` attribute with the
+  database data type that will be used in the
+  :class:`~django.db.models.functions.Cast` function for a ``CharField`` if the
+  ``max_length`` argument isn't provided.
+
 Dropped support for Oracle 11.2
 -------------------------------
 

tests/db_functions/test_cast.py

@@ -19,6 +19,10 @@ class CastTests(TestCase):
         numbers = Author.objects.annotate(cast_string=Cast('age', models.CharField(max_length=255)),)
         self.assertEqual(numbers.get().cast_string, '1')
 
+    def test_cast_to_char_field_without_max_length(self):
+        numbers = Author.objects.annotate(cast_string=Cast('age', models.CharField()))
+        self.assertEqual(numbers.get().cast_string, '1')
+
     # Silence "Truncated incorrect CHAR(1) value: 'Bob'".
     @ignore_warnings(module='django.db.backends.mysql.base')
     @skipUnlessDBFeature('supports_cast_with_precision')