Re-factor the ORM tests to use randomized fake data

- create `*Factory` classes with fakerboy and faker that generate
  randomized instances of the ORM models
- add new pytest marker: "db" are the integration tests involving the
  database whereas "e2e" will be all other integration tests
- streamline the docstrings in the ORM models

tests/db/test_customer.py

@@ -1,51 +1,35 @@
-"""Test the ORM's Customer model."""
+"""Test the ORM's `Customer` model."""
+# pylint:disable=no-self-use
 
 import pytest
-from sqlalchemy.orm import exc as orm_exc
 
 from urban_meal_delivery import db
 
 
 class TestSpecialMethods:
-    """Test special methods in Customer."""
+    """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']
+    def test_create_customer(self, customer):
+        """Test instantiation of a new `Customer` object."""
+        assert customer is not None
 
+    def test_text_representation(self, customer):
+        """`Customer` has a non-literal text representation."""
         result = repr(customer)
 
-        assert result == f'<Customer(#{id_})>'
+        assert result == f'<Customer(#{customer.id})>'
 
 
-@pytest.mark.e2e
+@pytest.mark.db
 @pytest.mark.no_cover
 class TestConstraints:
-    """Test the database constraints defined in Customer."""
+    """Test the database constraints defined in `Customer`."""
 
-    # pylint:disable=no-self-use
+    def test_insert_into_database(self, db_session, customer):
+        """Insert an instance into the (empty) database."""
+        assert db_session.query(db.Customer).count() == 0
 
-    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()
+        assert db_session.query(db.Customer).count() == 1