Add an ORM layer

- use SQLAlchemy (and PostgreSQL) to model the ORM layer
- add the following models:
  + Address => modelling all kinds of addresses
  + City => model the three target cities
  + Courier => model the UDP's couriers
  + Customer => model the UDP's customers
  + Order => model the orders received by the UDP
  + Restaurant => model the restaurants active on the UDP
- so far, the emphasis lies on expression the Foreign Key
  and Check Constraints that are used to validate the assumptions
  inherent to the cleanded data
- provide database-independent unit tests with 100% coverage
- provide additional integration tests ("e2e") that commit data to
  a PostgreSQL instance to validate that the constraints work
- adapt linting rules a bit

tests/db/test_couriers.py

@@ -0,0 +1,125 @@
+"""Test the ORM's Courier model."""
+
+import pytest
+from sqlalchemy import exc as sa_exc
+from sqlalchemy.orm import exc as orm_exc
+
+from urban_meal_delivery import db
+
+
+class TestSpecialMethods:
+    """Test special methods in Courier."""
+
+    # pylint:disable=no-self-use
+
+    def test_create_courier(self, courier_data):
+        """Test instantiation of a new Courier object."""
+        result = db.Courier(**courier_data)
+
+        assert result is not None
+
+    def test_text_representation(self, courier_data):
+        """Courier has a non-literal text representation."""
+        courier_data['id'] = 1
+        courier = db.Courier(**courier_data)
+        id_ = courier_data['id']
+
+        result = repr(courier)
+
+        assert result == f'<Courier(#{id_})>'
+
+
+@pytest.mark.e2e
+@pytest.mark.no_cover
+class TestConstraints:
+    """Test the database constraints defined in Courier."""
+
+    # pylint:disable=no-self-use
+
+    def test_insert_into_database(self, courier, db_session):
+        """Insert an instance into the database."""
+        db_session.add(courier)
+        db_session.commit()
+
+    def test_dublicate_primary_key(self, courier, courier_data, db_session):
+        """Can only add a record once."""
+        db_session.add(courier)
+        db_session.commit()
+
+        another_courier = db.Courier(**courier_data)
+        db_session.add(another_courier)
+
+        with pytest.raises(orm_exc.FlushError):
+            db_session.commit()
+
+    def test_invalid_vehicle(self, courier, db_session):
+        """Insert an instance with invalid data."""
+        courier.vehicle = 'invalid'
+        db_session.add(courier)
+
+        with pytest.raises(sa_exc.IntegrityError):
+            db_session.commit()
+
+    def test_negative_speed(self, courier, db_session):
+        """Insert an instance with invalid data."""
+        courier.historic_speed = -1
+        db_session.add(courier)
+
+        with pytest.raises(sa_exc.IntegrityError):
+            db_session.commit()
+
+    def test_unrealistic_speed(self, courier, db_session):
+        """Insert an instance with invalid data."""
+        courier.historic_speed = 999
+        db_session.add(courier)
+
+        with pytest.raises(sa_exc.IntegrityError):
+            db_session.commit()
+
+    def test_negative_capacity(self, courier, db_session):
+        """Insert an instance with invalid data."""
+        courier.capacity = -1
+        db_session.add(courier)
+
+        with pytest.raises(sa_exc.IntegrityError):
+            db_session.commit()
+
+    def test_too_much_capacity(self, courier, db_session):
+        """Insert an instance with invalid data."""
+        courier.capacity = 999
+        db_session.add(courier)
+
+        with pytest.raises(sa_exc.IntegrityError):
+            db_session.commit()
+
+    def test_negative_pay_per_hour(self, courier, db_session):
+        """Insert an instance with invalid data."""
+        courier.pay_per_hour = -1
+        db_session.add(courier)
+
+        with pytest.raises(sa_exc.IntegrityError):
+            db_session.commit()
+
+    def test_too_much_pay_per_hour(self, courier, db_session):
+        """Insert an instance with invalid data."""
+        courier.pay_per_hour = 9999
+        db_session.add(courier)
+
+        with pytest.raises(sa_exc.IntegrityError):
+            db_session.commit()
+
+    def test_negative_pay_per_order(self, courier, db_session):
+        """Insert an instance with invalid data."""
+        courier.pay_per_order = -1
+        db_session.add(courier)
+
+        with pytest.raises(sa_exc.IntegrityError):
+            db_session.commit()
+
+    def test_too_much_pay_per_order(self, courier, db_session):
+        """Insert an instance with invalid data."""
+        courier.pay_per_order = 999
+        db_session.add(courier)
+
+        with pytest.raises(sa_exc.IntegrityError):
+            db_session.commit()