Fix DROP index vs constraint issue (#38)

sql_server/pyodbc/schema.py

@@ -580,15 +580,21 @@ def _delete_unique_constraints(self, model, old_field, new_field, strict=False):
             unique_columns.append([old_field.column])
         if unique_columns:
             for columns in unique_columns:
-                constraint_names = self._constraint_names(model, columns, unique=True)
+                constraint_names_normal = self._constraint_names(model, columns, unique=True, index=False)
+                constraint_names_index = self._constraint_names(model, columns, unique=True, index=True)
+                constraint_names = constraint_names_normal + constraint_names_index
                 if strict and len(constraint_names) != 1:
                     raise ValueError("Found wrong number (%s) of unique constraints for %s.%s" % (
                         len(constraint_names),
                         model._meta.db_table,
                         old_field.column,
                     ))
-                for constraint_name in constraint_names:
+                for constraint_name in constraint_names_normal:
                     self.execute(self._delete_constraint_sql(self.sql_delete_unique, model, constraint_name))
+                # Unique indexes which are not table constraints must be deleted using the appropriate SQL.
+                # These may exist for example to enforce ANSI-compliant unique constraints on nullable columns.
+                for index_name in constraint_names_index:
+                    self.execute(self._delete_constraint_sql(self.sql_delete_index, model, index_name))
 
     def _rename_field_sql(self, table, old_field, new_field, new_type):
         new_type = self._set_field_new_type_null_status(old_field, new_type)

testapp/migrations/0002_test_unique_nullable_part1.py

@@ -0,0 +1,19 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('testapp', '0001_initial'),
+    ]
+
+    operations = [
+        # Create with a field that is unique *and* nullable so it is implemented with a filtered unique index.
+        migrations.CreateModel(
+            name='TestUniqueNullableModel',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('test_field', models.CharField(max_length=100, null=True, unique=True)),
+            ],
+        ),
+    ]

testapp/migrations/0003_test_unique_nullable_part2.py

@@ -0,0 +1,18 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('testapp', '0002_test_unique_nullable_part1'),
+    ]
+
+    operations = [
+        # Now remove the null=True to check this transition is correctly handled.
+        migrations.AlterField(
+            model_name='testuniquenullablemodel',
+            name='test_field',
+            field=models.CharField(default='', max_length=100, unique=True),
+            preserve_default=False,
+        ),
+    ]

testapp/models.py

@@ -41,3 +41,10 @@ class UUIDModel(models.Model):
 
     def __str__(self):
         return self.pk
+
+
+class TestUniqueNullableModel(models.Model):
+    # This field started off as unique=True *and* null=True so it is implemented with a filtered unique index
+    # Then it is made non-nullable by a subsequent migration, to check this is correctly handled (the index
+    # should be dropped, then a normal unique constraint should be added, now that the column is not nullable)
+    test_field = models.CharField(max_length=100, unique=True)