Migration that alters unique nullable field to non-nullable

This migration fails

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)