Add tests for the migrate command and fix a bug they exposed

tests/migrations/test_base.py

@@ -0,0 +1,39 @@
+from django.test import TestCase
+from django.db import connection
+
+
+class MigrationTestBase(TestCase):
+    """
+    Contains an extended set of asserts for testing migrations and schema operations.
+    """
+
+    def assertTableExists(self, table):
+        self.assertIn(table, connection.introspection.get_table_list(connection.cursor()))
+
+    def assertTableNotExists(self, table):
+        self.assertNotIn(table, connection.introspection.get_table_list(connection.cursor()))
+
+    def assertColumnExists(self, table, column):
+        self.assertIn(column, [c.name for c in connection.introspection.get_table_description(connection.cursor(), table)])
+
+    def assertColumnNotExists(self, table, column):
+        self.assertNotIn(column, [c.name for c in connection.introspection.get_table_description(connection.cursor(), table)])
+
+    def assertColumnNull(self, table, column):
+        self.assertEqual([c.null_ok for c in connection.introspection.get_table_description(connection.cursor(), table) if c.name == column][0], True)
+
+    def assertColumnNotNull(self, table, column):
+        self.assertEqual([c.null_ok for c in connection.introspection.get_table_description(connection.cursor(), table) if c.name == column][0], False)
+
+    def assertIndexExists(self, table, columns, value=True):
+        self.assertEqual(
+            value,
+            any(
+                c["index"]
+                for c in connection.introspection.get_constraints(connection.cursor(), table).values()
+                if c['columns'] == list(columns)
+            ),
+        )
+
+    def assertIndexNotExists(self, table, columns):
+        return self.assertIndexExists(table, columns, False)