Refs #28816 -- Prevented silencing data loss when decreasing CharField.max_length for ArrayField.base_field on PostgreSQL.

django/db/backends/postgresql/schema.py

@@ -47,6 +47,13 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
             field.db_type(self.connection),
         )
 
+    def _field_base_data_types(self, field):
+        # Yield base data types for array fields.
+        if field.base_field.get_internal_type() == 'ArrayField':
+            yield from self._field_base_data_types(field.base_field)
+        else:
+            yield self._field_data_type(field.base_field)
+
     def _create_like_index_sql(self, model, field):
         """
         Return the statement to create an index with varchar operator pattern
@@ -72,8 +79,15 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
     def _alter_column_type_sql(self, model, old_field, new_field, new_type):
         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):
+        using_sql = ' USING %(column)s::%(type)s'
-            self.sql_alter_column_type += ' USING %(column)s::%(type)s'
+        new_internal_type = new_field.get_internal_type()
+        old_internal_type = old_field.get_internal_type()
+        if new_internal_type == 'ArrayField' and new_internal_type == old_internal_type:
+            # Compare base data types for array fields.
+            if list(self._field_base_data_types(old_field)) != list(self._field_base_data_types(new_field)):
+                self.sql_alter_column_type += using_sql
+        elif self._field_data_type(old_field) != self._field_data_type(new_field):
+            self.sql_alter_column_type += using_sql
         # Make ALTER TYPE with SERIAL make sense.
         table = strip_quotes(model._meta.db_table)
         serial_fields_map = {'bigserial': 'bigint', 'serial': 'integer', 'smallserial': 'smallint'}

tests/schema/tests.py

@@ -895,6 +895,54 @@ class SchemaTests(TransactionTestCase):
         with connection.schema_editor() as editor:
             editor.alter_field(Foo, old_field, new_field, strict=True)
 
+    @isolate_apps('schema')
+    @unittest.skipUnless(connection.vendor == 'postgresql', 'PostgreSQL specific')
+    def test_alter_array_field_decrease_base_field_length(self):
+        from django.contrib.postgres.fields import ArrayField
+
+        class ArrayModel(Model):
+            field = ArrayField(CharField(max_length=16))
+
+            class Meta:
+                app_label = 'schema'
+
+        with connection.schema_editor() as editor:
+            editor.create_model(ArrayModel)
+        self.isolated_local_models = [ArrayModel]
+        ArrayModel.objects.create(field=['x' * 16])
+        old_field = ArrayModel._meta.get_field('field')
+        new_field = ArrayField(CharField(max_length=15))
+        new_field.set_attributes_from_name('field')
+        new_field.model = ArrayModel
+        with connection.schema_editor() as editor:
+            msg = 'value too long for type character varying(15)'
+            with self.assertRaisesMessage(DataError, msg):
+                editor.alter_field(ArrayModel, old_field, new_field, strict=True)
+
+    @isolate_apps('schema')
+    @unittest.skipUnless(connection.vendor == 'postgresql', 'PostgreSQL specific')
+    def test_alter_array_field_decrease_nested_base_field_length(self):
+        from django.contrib.postgres.fields import ArrayField
+
+        class ArrayModel(Model):
+            field = ArrayField(ArrayField(CharField(max_length=16)))
+
+            class Meta:
+                app_label = 'schema'
+
+        with connection.schema_editor() as editor:
+            editor.create_model(ArrayModel)
+        self.isolated_local_models = [ArrayModel]
+        ArrayModel.objects.create(field=[['x' * 16]])
+        old_field = ArrayModel._meta.get_field('field')
+        new_field = ArrayField(ArrayField(CharField(max_length=15)))
+        new_field.set_attributes_from_name('field')
+        new_field.model = ArrayModel
+        with connection.schema_editor() as editor:
+            msg = 'value too long for type character varying(15)'
+            with self.assertRaisesMessage(DataError, msg):
+                editor.alter_field(ArrayModel, old_field, new_field, strict=True)
+
     def test_alter_textfield_to_null(self):
         """
         #24307 - Should skip an alter statement on databases with