TDD: tests that fail due to ESSolutions#58

Doubtless there are more cases that should be tested, but
these are the 3 specific ones that I've found to be broken
and reported in issue ESSolutions#58

These tests fail on latest django-mssql-backend (e.g. v2.8.1)
but passed on v2.4.2 before ESSolutions#24 went in.

testapp/migrations/0008_test_indexes_retained_part1.py

@@ -0,0 +1,21 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('testapp', '0007_test_remove_onetoone_field_part2'),
+    ]
+
+    # Issue #58 test prep
+    operations = [
+        migrations.CreateModel(
+            name='TestIndexesRetained',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('a', models.IntegerField(db_index=True)),
+                ('b', models.IntegerField(db_index=True)),
+                ('c', models.IntegerField(db_index=True)),
+            ],
+        ),
+    ]

testapp/migrations/0009_test_indexes_retained_part2.py

@@ -0,0 +1,26 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('testapp', '0008_test_indexes_retained_part1'),
+    ]
+
+    # Issue #58 test operations which should leave index intact
+    operations = [
+        migrations.AlterField(
+            model_name='testindexesretained',
+            name='a',
+            field=models.IntegerField(db_index=True, null=True),
+        ),
+        migrations.RenameField(
+            model_name='testindexesretained',
+            old_name='b',
+            new_name='b_renamed',
+        ),
+        migrations.RenameModel(
+            old_name='TestIndexesRetained',
+            new_name='TestIndexesRetainedRenamed',
+        ),
+    ]

testapp/models.py

@@ -67,7 +67,17 @@ class Meta:
 
 
 class TestRemoveOneToOneFieldModel(models.Model):
-    # Fields used for testing removing OneToOne field. Verifies that delete_unique do not try to remove indexes
-    # thats already is removed.
+    # Fields used for testing removing OneToOne field. Verifies that delete_unique
+    # does not try to remove indexes that have already been removed (Issue #51)
     # b = models.OneToOneField('self', on_delete=models.SET_NULL, null=True)
     a = models.CharField(max_length=50)
+
+
+class TestIndexesRetainedRenamed(models.Model):
+    # Issue #58 - in all these cases the column index should still exist afterwards
+    # case (a) `a` starts out not nullable, but then is changed to be nullable
+    a = models.IntegerField(db_index=True, null=True)
+    # case (b) column originally called `b` is renamed
+    b_renamed = models.IntegerField(db_index=True)
+    # case (c) this entire model is renamed - this is just a column whose index can be checked afterwards
+    c = models.IntegerField(db_index=True)

testapp/tests/test_indexes.py

@@ -0,0 +1,47 @@
+import django.db
+from django.test import TestCase
+
+from ..models import (
+    TestIndexesRetainedRenamed
+)
+
+
+class TestIndexesRetained(TestCase):
+    """
+    Indexes dropped during a migration should be re-created afterwards
+    assuming the field still has `db_index=True` (issue #58)
+    """
+
+    @classmethod
+    def setUpClass(cls):
+        super().setUpClass()
+        # Pre-fetch which indexes exist for the relevant test model
+        # now that all the test migrations have run
+        connection = django.db.connections[django.db.DEFAULT_DB_ALIAS]
+        cls.constraints = connection.introspection.get_constraints(
+            connection.cursor(),
+            table_name=TestIndexesRetainedRenamed._meta.db_table
+        )
+        cls.indexes = {k: v for k, v in cls.constraints.items() if v['index'] is True}
+
+    def _assert_index_exists(self, columns):
+        matching = {k: v for k, v in self.indexes.items() if set(v['columns']) == columns}
+        assert len(matching) == 1, (
+            "Expected 1 index for columns %s but found %d %s" % (
+                columns,
+                len(matching),
+                ', '.join(matching.keys())
+            )
+        )
+
+    def test_field_made_nullable(self):
+        # Issue #58 case (a)
+        self._assert_index_exists({'a'})
+
+    def test_field_renamed(self):
+        # Issue #58 case (b)
+        self._assert_index_exists({'b_renamed'})
+
+    def test_table_renamed(self):
+        # Issue #58 case (c)
+        self._assert_index_exists({'c'})