Creating a unique nullable field and then altering it to be non-nullable no longer produces an error

sql_server/pyodbc/schema.py

@@ -18,6 +18,8 @@
 from django.db.transaction import TransactionManagementError
 from django.utils.encoding import force_str
 
+from collections import defaultdict
+
 
 class Statement(DjStatement):
     def __hash__(self):
@@ -68,6 +70,8 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
     sql_create_unique_null = "CREATE UNIQUE INDEX %(name)s ON %(table)s(%(columns)s) " \
                              "WHERE %(columns)s IS NOT NULL"
 
+    _deferred_unique_indexes = defaultdict(list)
+
     def _alter_column_default_sql(self, model, old_field, new_field, drop=False):
         """
         Hook to specialize column default alteration.
@@ -236,6 +240,15 @@ def alter_db_table(self, model, old_db_table, new_db_table):
 
         return super().alter_db_table(model, old_db_table, new_db_table)
 
+    def _delete_deferred_unique_indexes_for_field(self, field):
+        deferred_statements = self._deferred_unique_indexes.get(str(field), [])
+        for stmt in deferred_statements:
+            if stmt in self.deferred_sql:
+                self.deferred_sql.remove(stmt)
+
+    def _add_deferred_unique_index_for_field(self, field, statement):
+        self._deferred_unique_indexes[str(field)].append(statement)
+
     def _alter_field(self, model, old_field, new_field, old_type, new_type,
                      old_db_params, new_db_params, strict=False):
         """Actually perform a "physical" (non-ManyToMany) field update."""
@@ -449,6 +462,7 @@ def _alter_field(self, model, old_field, new_field, old_type, new_type,
                 )
             else:
                 self.execute(self._create_unique_sql(model, [new_field.column]))
+            self._delete_deferred_unique_indexes_for_field(new_field)
         # Added an index?
         # constraint will no longer be used in lieu of an index. The following
         # lines from the truth table show all True cases; the rest are False:
@@ -477,6 +491,7 @@ def _alter_field(self, model, old_field, new_field, old_type, new_type,
                     )
                 else:
                     self.execute(self._create_unique_sql(model, columns=[old_field.column]))
+                self._delete_deferred_unique_indexes_for_field(old_field)
             else:
                 for fields in model._meta.unique_together:
                     columns = [model._meta.get_field(field).column for field in fields]
@@ -644,9 +659,11 @@ def add_field(self, model, field):
                 not field.many_to_many and field.null and field.unique):
 
             definition = definition.replace(' UNIQUE', '')
-            self.deferred_sql.append(self._create_index_sql(
+            statement = self._create_index_sql(
                 model, [field], sql=self.sql_create_unique_null, suffix="_uniq"
-            ))
+            )
+            self.deferred_sql.append(statement)
+            self._add_deferred_unique_index_for_field(field, statement)
 
         # Check constraints can go on the column SQL here
         db_params = field.db_parameters(connection=self.connection)
@@ -750,9 +767,11 @@ def create_model(self, model):
                     not field.many_to_many and field.null and field.unique):
 
                 definition = definition.replace(' UNIQUE', '')
-                self.deferred_sql.append(self._create_index_sql(
+                statement = self._create_index_sql(
                     model, [field], sql=self.sql_create_unique_null, suffix="_uniq"
-                ))
+                )
+                self.deferred_sql.append(statement)
+                self._add_deferred_unique_index_for_field(field, statement)
 
             # Check constraints can go on the column SQL here
             db_params = field.db_parameters(connection=self.connection)

testapp/migrations/0008_test_alter_nullable_in_unique_field.py

@@ -0,0 +1,24 @@
+# Generated by Django 3.0.4 on 2020-04-20 14:59
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('testapp', '0007_test_remove_onetoone_field_part2'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='TestAlterNullableInUniqueField',
+            fields=[
+                ('a', models.CharField(max_length=50, null=True, unique=True)),
+            ],
+        ),
+        migrations.AlterField(
+            model_name='testalternullableinuniquefield',
+            name='a',
+            field=models.CharField(max_length=50, unique=True),
+        )
+    ]

testapp/models.py

@@ -71,3 +71,11 @@ class TestRemoveOneToOneFieldModel(models.Model):
     # thats already is removed.
     # b = models.OneToOneField('self', on_delete=models.SET_NULL, null=True)
     a = models.CharField(max_length=50)
+
+
+class TestAlterNullableInUniqueField(models.Model):
+    """ Model used to test a single migration that creates a field with unique=True and null=True and then alters
+    the field to set null=False. This is a common use case when you want to add a non-nullable unique field to a
+    pre-existing model. In order to make that work you need to first create the unique field as nullable, then
+    populate the field for every pre-existing instance, and then alter the field to set it to non-nullaable. """
+    a = models.CharField(max_length=50, unique=True, null=True)