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_cities.py

@@ -0,0 +1,99 @@
+"""Test the ORM's City model."""
+
+import pytest
+from sqlalchemy.orm import exc as orm_exc
+
+from urban_meal_delivery import db
+
+
+class TestSpecialMethods:
+    """Test special methods in City."""
+
+    # pylint:disable=no-self-use
+
+    def test_create_city(self, city_data):
+        """Test instantiation of a new City object."""
+        result = db.City(**city_data)
+
+        assert result is not None
+
+    def test_text_representation(self, city_data):
+        """City has a non-literal text representation."""
+        city = db.City(**city_data)
+        name = city_data['name']
+
+        result = repr(city)
+
+        assert result == f'<City({name})>'
+
+
+@pytest.mark.e2e
+@pytest.mark.no_cover
+class TestConstraints:
+    """Test the database constraints defined in City."""
+
+    # pylint:disable=no-self-use
+
+    def test_insert_into_database(self, city, db_session):
+        """Insert an instance into the database."""
+        db_session.add(city)
+        db_session.commit()
+
+    def test_dublicate_primary_key(self, city, city_data, db_session):
+        """Can only add a record once."""
+        db_session.add(city)
+        db_session.commit()
+
+        another_city = db.City(**city_data)
+        db_session.add(another_city)
+
+        with pytest.raises(orm_exc.FlushError):
+            db_session.commit()
+
+
+class TestProperties:
+    """Test properties in City."""
+
+    # pylint:disable=no-self-use
+
+    def test_location_data(self, city_data):
+        """Test City.location property."""
+        city = db.City(**city_data)
+
+        result = city.location
+
+        assert isinstance(result, dict)
+        assert len(result) == 2
+        assert result['latitude'] == pytest.approx(city_data['_center_latitude'])
+        assert result['longitude'] == pytest.approx(city_data['_center_longitude'])
+
+    def test_viewport_data_overall(self, city_data):
+        """Test City.viewport property."""
+        city = db.City(**city_data)
+
+        result = city.viewport
+
+        assert isinstance(result, dict)
+        assert len(result) == 2
+
+    def test_viewport_data_northeast(self, city_data):
+        """Test City.viewport property."""
+        city = db.City(**city_data)
+
+        result = city.viewport['northeast']
+
+        assert isinstance(result, dict)
+        assert len(result) == 2
+        assert result['latitude'] == pytest.approx(city_data['_northeast_latitude'])
+        assert result['longitude'] == pytest.approx(city_data['_northeast_longitude'])
+
+    def test_viewport_data_southwest(self, city_data):
+        """Test City.viewport property."""
+        city = db.City(**city_data)
+
+        result = city.viewport['southwest']
+
+        assert isinstance(result, dict)
+        assert len(result) == 2
+        assert result['latitude'] == pytest.approx(city_data['_southwest_latitude'])
+        assert result['longitude'] == pytest.approx(city_data['_southwest_longitude'])