added some tests

tests/test_mapping_correctness.py

@@ -0,0 +1,26 @@
+import unittest
+
+from slack_backup import db
+from slack_backup import objects as o
+
+
+class TestMapping(unittest.TestCase):
+
+    def test_users(self):
+        db.connect()
+        session = db.Session()
+
+        users = session.query(o.User).all()
+        self.assertListEqual(users, [])
+
+        session.add(o.User())
+
+        users = session.query(o.User).all()
+        self.assertEqual(len(users), 1)
+
+        user = users[0]
+
+        self.assertListEqual(user.channels, [])
+        self.assertListEqual(user.topics, [])
+        self.assertListEqual(user.purposes, [])
+        self.assertEqual(user.id, 1)