Fixed #28816 -- Prevented silencing data loss when decreasing CharField.max_length on PostgreSQL.

django/db/backends/postgresql/schema.py

@@ -6,8 +6,6 @@ from django.db.backends.ddl_references import IndexColumns
 
 class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
 
-    sql_alter_column_type = "ALTER COLUMN %(column)s TYPE %(type)s USING %(column)s::%(type)s"
-
     sql_create_sequence = "CREATE SEQUENCE %(sequence)s"
     sql_delete_sequence = "DROP SEQUENCE IF EXISTS %(sequence)s CASCADE"
     sql_set_sequence_max = "SELECT setval('%(sequence)s', MAX(%(column)s)) FROM %(table)s"
@@ -36,6 +34,11 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
             output.append(like_index_statement)
         return output
 
+    def _field_data_type(self, field):
+        if field.is_relation:
+            return field.rel_db_type(self.connection)
+        return self.connection.data_types[field.get_internal_type()]
+
     def _create_like_index_sql(self, model, field):
         """
         Return the statement to create an index with varchar operator pattern
@@ -59,7 +62,11 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         return None
 
     def _alter_column_type_sql(self, model, old_field, new_field, new_type):
-        """Make ALTER TYPE with SERIAL make sense."""
+        self.sql_alter_column_type = 'ALTER COLUMN %(column)s TYPE %(type)s'
+        # Cast when data type changed.
+        if self._field_data_type(old_field) != self._field_data_type(new_field):
+            self.sql_alter_column_type += ' USING %(column)s::%(type)s'
+        # Make ALTER TYPE with SERIAL make sense.
         table = model._meta.db_table
         if new_type.lower() in ("serial", "bigserial"):
             column = new_field.column

tests/schema/tests.py

@@ -21,6 +21,7 @@ from django.db.models.fields.related import (
 )
 from django.db.models.indexes import Index
 from django.db.transaction import TransactionManagementError, atomic
+from django.db.utils import DataError
 from django.test import (
     TransactionTestCase, skipIfDBFeature, skipUnlessDBFeature,
 )
@@ -803,6 +804,21 @@ class SchemaTests(TransactionTestCase):
         with connection.schema_editor() as editor:
             editor.alter_field(Author, old_field, new_field, strict=True)
 
+    @unittest.skipUnless(connection.vendor == 'postgresql', 'PostgreSQL specific')
+    def test_alter_char_field_decrease_length(self):
+        # Create the table.
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+        Author.objects.create(name='x' * 255)
+        # Change max_length of CharField.
+        old_field = Author._meta.get_field('name')
+        new_field = CharField(max_length=254)
+        new_field.set_attributes_from_name('name')
+        with connection.schema_editor() as editor:
+            msg = 'value too long for type character varying(254)'
+            with self.assertRaisesMessage(DataError, msg):
+                editor.alter_field(Author, old_field, new_field, strict=True)
+
     def test_alter_textfield_to_null(self):
         """
         #24307 - Should skip an alter statement on databases with