fix db error on second login

ki/models.py

@@ -13,7 +13,7 @@ class User(db.Model):
     auth_id = Column(String(50), nullable=False, unique=True)
     profile_id = Column(Integer, ForeignKey("profile.id"), nullable=True)
 
-    tokens = relationship("Token", uselist=False, back_populates="user")
+    tokens = relationship("Token", back_populates="user")
     profile = relationship("Profile", back_populates="user")
 
     def to_dict(self):

ki/test/test_login_endpoint.py

@@ -0,0 +1,39 @@
+from alembic import command
+import json
+import unittest
+
+from app import app, migrate
+from ki.commands import seed
+
+
+class TestLoginEndpoint(unittest.TestCase):
+    def setUp(self):
+        app.debug = True
+        app.config["SQLALCHEMY_DATABASE_URI"] = "sqlite:///:memory:"
+        self.client = app.test_client()
+
+        with app.app_context():
+            config = migrate.get_config()
+            command.upgrade(config, "head")
+
+        seed()
+
+    def test_login(self):
+        response1_data = self.login()
+        response2_data = self.login()
+        self.assertNotEqual(response1_data["token"], response2_data["token"])
+
+    def login(self):
+        response = self.client.post("/users/login",
+                                    data=json.dumps({
+                                        "username": "peter",
+                                        "password": "geheim"
+                                    }),
+                                    content_type="application/json")
+        self.assertEqual(response.status_code, 200)
+        self.assertIn("token", response.json)
+        return response.json
+
+
+if __name__ == "main":
+    unittest.main()