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_customer.py

@@ -0,0 +1,51 @@
+"""Test the ORM's Customer model."""
+
+import pytest
+from sqlalchemy.orm import exc as orm_exc
+
+from urban_meal_delivery import db
+
+
+class TestSpecialMethods:
+    """Test special methods in Customer."""
+
+    # pylint:disable=no-self-use
+
+    def test_create_customer(self, customer_data):
+        """Test instantiation of a new Customer object."""
+        result = db.Customer(**customer_data)
+
+        assert result is not None
+
+    def test_text_representation(self, customer_data):
+        """Customer has a non-literal text representation."""
+        customer = db.Customer(**customer_data)
+        id_ = customer_data['id']
+
+        result = repr(customer)
+
+        assert result == f'<Customer(#{id_})>'
+
+
+@pytest.mark.e2e
+@pytest.mark.no_cover
+class TestConstraints:
+    """Test the database constraints defined in Customer."""
+
+    # pylint:disable=no-self-use
+
+    def test_insert_into_database(self, customer, db_session):
+        """Insert an instance into the database."""
+        db_session.add(customer)
+        db_session.commit()
+
+    def test_dublicate_primary_key(self, customer, customer_data, db_session):
+        """Can only add a record once."""
+        db_session.add(customer)
+        db_session.commit()
+
+        another_customer = db.Customer(**customer_data)
+        db_session.add(another_customer)
+
+        with pytest.raises(orm_exc.FlushError):
+            db_session.commit()