From 0b0743700a319b09f638c273c69bd7fc69b3bf94 Mon Sep 17 00:00:00 2001
From: "pavdwest.xps15" <pavdwest.xps15>
Date: Sun, 6 Sep 2026 17:17:47 +0200
Subject: [PATCH] Don't crash when an altered column's old class is a custom
 Column subclass
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`SchemaDiffer.alter_columns` looks up `UniqueGlobalNames.COLUMN_<NAME>` for
both the new and the old column class, to spot an import that would conflict
with a name the generated migration already uses. The lookup for the new class
passes a `None` default; the one for the old class does not, so altering a
column whose previous class was a user-defined `Column` subclass raises

    AttributeError: type object 'UniqueGlobalNames' has no attribute
    'COLUMN_CUSTOMVARCHAR'

and `piccolo migrations new --auto` dies before writing anything.

`UniqueGlobalNames` only carries the built-in column classes, so a miss is the
expected outcome for any subclass a user defines — which is a supported thing
to do, and common for projects that wrap a column type to add a default or
narrow its type. Five of the six lookups in this module already pass `None`;
this makes the sixth behave the same way.

Closes #1428
---
 piccolo/apps/migrations/auto/schema_differ.py |  1 +
 .../migrations/auto/test_schema_differ.py     | 47 +++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/piccolo/apps/migrations/auto/schema_differ.py b/piccolo/apps/migrations/auto/schema_differ.py
index 58543cbca..cece5334b 100644
--- a/piccolo/apps/migrations/auto/schema_differ.py
+++ b/piccolo/apps/migrations/auto/schema_differ.py
@@ -516,6 +516,7 @@ def alter_columns(self) -> AlterStatements:
                             expect_conflict_with_global_name=getattr(
                                 UniqueGlobalNames,
                                 f"COLUMN_{alter_column.old_column_class.__name__.upper()}",  # noqa: E501
+                                None,
                             ),
                         )
                     )
diff --git a/tests/apps/migrations/auto/test_schema_differ.py b/tests/apps/migrations/auto/test_schema_differ.py
index 64a1f2d67..5b72cd3ef 100644
--- a/tests/apps/migrations/auto/test_schema_differ.py
+++ b/tests/apps/migrations/auto/test_schema_differ.py
@@ -450,6 +450,53 @@ def test_alter_column_precision(self) -> None:
             "manager.alter_column(table_class_name='Ticket', tablename='ticket', column_name='price', db_column_name='price', params={'digits': (4, 2)}, old_params={'digits': (5, 2)}, column_class=Numeric, old_column_class=Numeric, schema=None)",  # noqa
         )
 
+    def test_alter_column_with_custom_old_column_class(self) -> None:
+        """
+        A column whose *old* class is a user-defined ``Column`` subclass must
+        not crash the differ.
+
+        ``UniqueGlobalNames`` only knows the built-in column classes, so the
+        lookup for the old class has to tolerate a miss the same way the lookup
+        for the new class already does.
+
+        https://github.com/piccolo-orm/piccolo/issues/1428
+
+        """
+
+        class CustomVarchar(Varchar):
+            pass
+
+        name_1 = Varchar(length=20)
+        name_1._meta.name = "name"
+
+        name_2 = CustomVarchar(length=10)
+        name_2._meta.name = "name"
+
+        schema: list[DiffableTable] = [
+            DiffableTable(
+                class_name="Band",
+                tablename="band",
+                columns=[name_1],
+            )
+        ]
+        schema_snapshot: list[DiffableTable] = [
+            DiffableTable(
+                class_name="Band",
+                tablename="band",
+                columns=[name_2],
+            )
+        ]
+
+        schema_differ = SchemaDiffer(
+            schema=schema, schema_snapshot=schema_snapshot, auto_input="y"
+        )
+
+        self.assertEqual(len(schema_differ.alter_columns.statements), 1)
+        self.assertIn(
+            "old_column_class=CustomVarchar",
+            schema_differ.alter_columns.statements[0],
+        )
+
     def test_db_column_name(self) -> None:
         """
         Make sure alter statements use the ``db_column_name`` if provided.