Fixed a small bug in the SQL queries and the unit tests

matemat/db/facade.py

@@ -606,7 +606,7 @@ class MatematDatabase(object):
         transactions: List[Transaction] = []
         with self.db.transaction() as cursor:
             cursor.execute('''
-            SELECT COALESCE(MAX(r.date), u.created), MAX(r.last_ta_id)
+            SELECT COALESCE(MAX(r.date), u.created), COALESCE(MAX(r.last_ta_id), 0)
             FROM users AS u
             LEFT JOIN receipts AS r
               ON r.user_id = u.user_id

matemat/db/test/test_facade.py

@@ -506,14 +506,14 @@ class DatabaseTest(unittest.TestCase):
             db.increment_consumption(user, product)
             db.deposit(user, 1337)
             receipt1: Receipt = db.create_receipt(user, write=True)
-            # Attempt to create a receipt with zero transactions. Won't be written to DB.
+            # Attempt to create a receipt with zero transactions. Will carry NULL transaction IDs
             receipt2: Receipt = db.create_receipt(user, write=True)
-            self.assertEqual(-1, receipt2.id)
+            self.assertNotEqual(-1, receipt2.id)
             self.assertEqual(0, len(receipt2.transactions))
 
             with db.transaction() as c:
                 c.execute('SELECT COUNT(receipt_id) FROM receipts')
-                self.assertEqual(1, c.fetchone()[0])
+                self.assertEqual(2, c.fetchone()[0])
 
             db.increment_consumption(user, product)
             db.change_user(user, agent=admin, balance=4200)
@@ -529,7 +529,7 @@ class DatabaseTest(unittest.TestCase):
 
             with db.transaction() as c:
                 c.execute('SELECT COUNT(receipt_id) FROM receipts')
-                self.assertEqual(1, c.fetchone()[0])
+                self.assertEqual(2, c.fetchone()[0])
 
             self.assertEqual(user, receipt1.user)
             self.assertEqual(3, len(receipt1.transactions))