Fix for issue #1301

piccolo/apps/migrations/auto/migration_manager.py

@@ -546,11 +546,12 @@ async def _run_alter_columns(self, backwards: bool = False):
                     constraint_name = await get_fk_constraint_name(
                         column=fk_column
                     )
-                    await self._run_query(
-                        _Table.alter().drop_constraint(
-                            constraint_name=constraint_name
+                    if constraint_name:
+                        await self._run_query(
+                            _Table.alter().drop_constraint(
+                                constraint_name=constraint_name
+                            )
                         )
-                    )
 
                     # Then add a new foreign key constraint
                     await self._run_query(

piccolo/query/constraints.py

@@ -1,10 +1,11 @@
 from dataclasses import dataclass
+from typing import Optional
 
 from piccolo.columns import ForeignKey
 from piccolo.columns.base import OnDelete, OnUpdate
 
 
-async def get_fk_constraint_name(column: ForeignKey) -> str:
+async def get_fk_constraint_name(column: ForeignKey) -> Optional[str]:
     """
     Checks what the foreign key constraint is called in the database.
     """
@@ -40,7 +41,10 @@ async def get_fk_constraint_name(column: ForeignKey) -> str:
         column_name,
     )
 
-    return constraints[0]["fk_constraint_name"]
+    # if we change the column type from a non-FK column to
+    # an FK column, the previous column type has no FK constraints
+    # and we skip this to allow the migration to continue
+    return constraints[0]["fk_constraint_name"] if constraints else None
 
 
 @dataclass