Fix SaferAlterFieldSetNotNull for custom-named fields

Prior to this change, this operation didn't work for fields that had
column names in the database that didn't match their names (such as
ForeignKey fields, which have an "_id" suffix).

This commit changes that by obtaining the column name from the model's
meta class instead.

CHANGELOG.md

@@ -8,6 +8,13 @@ Changelog](https://keepachangelog.com/en/1.1.0/), and this project adheres to
 
 ## [Unreleased]
 
+### Fixed
+
+- `SaferAlterFieldSetNotNull` did not work when the field dropping the NOT NULL
+  constraint was a ForeignKey. An error such as:
+  `django.db.utils.ProgrammingError: column "foo" does not exist` would have
+  been raised instead.
+
 ## [0.1.17] - 2025-01-14
 
 ### Added

src/django_pg_migration_tools/operations.py

@@ -1111,11 +1111,12 @@ def database_forwards(
         from_state: migrations.state.ProjectState,
         to_state: migrations.state.ProjectState,
     ) -> None:
+        model = to_state.apps.get_model(app_label, self.model_name)
         NullsManager().set_not_null(
             app_label,
             schema_editor,
-            model=to_state.apps.get_model(app_label, self.model_name),
-            column_name=self.name,
+            model=model,
+            column_name=model._meta.get_field(self.name).column,
         )
 
     def database_backwards(
@@ -1125,11 +1126,12 @@ def database_backwards(
         from_state: migrations.state.ProjectState,
         to_state: migrations.state.ProjectState,
     ) -> None:
+        model = to_state.apps.get_model(app_label, self.model_name)
         NullsManager().set_null(
             app_label,
             schema_editor,
-            model=to_state.apps.get_model(app_label, self.model_name),
-            column_name=self.name,
+            model=model,
+            column_name=model._meta.get_field(self.name).column,
         )
 
     def describe(self) -> str:

tests/django_pg_migration_tools/test_operations.py

@@ -1693,7 +1693,6 @@ def test_when_not_allowed_to_migrate_by_the_router(self):
         # the router.
         assert len(queries) == 0
 
-    @pytest.mark.xfail
     @pytest.mark.django_db(transaction=True)
     def test_operation(self):
         project_state = ProjectState()