Adds unit test for issues #110 and #90 (#115)

* Added test to test changing from non-nullable to nullable with unique
index

* Fixed failing unit test on Django 3.1 and lower due to bad import

Author: jmah8

testapp/migrations/0018_choice_question.py

@@ -0,0 +1,34 @@
+# Generated by Django 3.2.12 on 2022-03-14 18:36
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('testapp', '0017_binarydata_testcheckconstraintwithunicode_and_more'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Question',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('question_text', models.CharField(max_length=200)),
+                ('pub_date', models.DateTimeField(verbose_name='date published')),
+            ],
+        ),
+        migrations.CreateModel(
+            name='Choice',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('choice_text', models.CharField(max_length=200)),
+                ('votes', models.IntegerField(default=0)),
+                ('question', models.ForeignKey(null=True, on_delete=django.db.models.deletion.CASCADE, to='testapp.question')),
+            ],
+            options={
+                'unique_together': {('question', 'choice_text')},
+            },
+        ),
+    ]

testapp/models.py

@@ -180,3 +180,23 @@ class Meta:
                     name='name_does_not_starts_with_\u00f7',
                 )
             ]
+
+
+class Question(models.Model):
+    question_text = models.CharField(max_length=200)
+    pub_date = models.DateTimeField('date published')
+
+    def __str__(self):
+        return self.question_text
+
+    def was_published_recently(self):
+        return self.pub_date >= timezone.now() - datetime.timedelta(days=1)
+
+
+class Choice(models.Model):
+    question = models.ForeignKey(Question, on_delete=models.CASCADE, null=True)
+    choice_text = models.CharField(max_length=200)
+    votes = models.IntegerField(default=0)
+
+    class Meta:
+        unique_together = (('question', 'choice_text'))

testapp/tests/test_indexes.py

@@ -1,14 +1,27 @@
 import logging
 
 import django.db
+from django import VERSION
 from django.apps import apps
+from django.db import models
 from django.db.models import UniqueConstraint
+from django.db.utils import DEFAULT_DB_ALIAS, ConnectionHandler, ProgrammingError
 from django.test import TestCase
 
 from ..models import (
-    TestIndexesRetainedRenamed
+    TestIndexesRetainedRenamed,
+    Choice,
+    Question,
 )
 
+connections = ConnectionHandler()
+
+if (VERSION >= (3, 2)):
+    from django.utils.connection import ConnectionProxy
+    connection = ConnectionProxy(connections, DEFAULT_DB_ALIAS)
+else:
+    from django.db import DefaultConnectionProxy
+    connection = DefaultConnectionProxy()
 
 logger = logging.getLogger('mssql.tests')
 
@@ -139,3 +152,33 @@ def test_correct_indexes_exist(self):
                         '\n'.join(str(i) for i in col_indexes),
                     )
                 logger.debug('  Found %s index(es) as expected', len(col_indexes))
+
+
+class TestIndexesBeingDropped(TestCase):
+
+    def test_unique_index_dropped(self):
+        """
+        Issues https://github.com/microsoft/mssql-django/issues/110
+        and https://github.com/microsoft/mssql-django/issues/90
+        Unique indexes not being dropped when changing non-nullable
+        foreign key with unique_together to nullable causing
+        dependent on column error
+        """
+        old_field = Choice._meta.get_field('question')
+        new_field = models.ForeignKey(
+            Question, null=False, on_delete=models.deletion.CASCADE
+        )
+        new_field.set_attributes_from_name("question")
+        with connection.schema_editor() as editor:
+            editor.alter_field(Choice, old_field, new_field, strict=True)
+
+        old_field = new_field
+        new_field = models.ForeignKey(
+            Question, null=True, on_delete=models.deletion.CASCADE
+        )
+        new_field.set_attributes_from_name("question")
+        try:
+            with connection.schema_editor() as editor:
+                editor.alter_field(Choice, old_field, new_field, strict=True)
+        except ProgrammingError:
+            self.fail("Unique indexes not being dropped")