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

src/urban_meal_delivery/db/restaurants.py

@@ -0,0 +1,42 @@
+"""Provide the ORM's Restaurant model."""
+
+import sqlalchemy as sa
+from sqlalchemy import orm
+
+from urban_meal_delivery.db import meta
+
+
+class Restaurant(meta.Base):
+    """A Restaurant selling meals on the UDP."""
+
+    # pylint:disable=too-few-public-methods
+
+    __tablename__ = 'restaurants'
+
+    # Columns
+    id = sa.Column(  # noqa:WPS125
+        sa.SmallInteger, primary_key=True, autoincrement=False,
+    )
+    created_at = sa.Column(sa.DateTime, nullable=False)
+    name = sa.Column(sa.Unicode(length=45), nullable=False)  # noqa:WPS432
+    _address_id = sa.Column('address_id', sa.Integer, nullable=False, index=True)
+    estimated_prep_duration = sa.Column(sa.SmallInteger, nullable=False)
+
+    # Constraints
+    __table_args__ = (
+        sa.ForeignKeyConstraint(
+            ['address_id'], ['addresses.id'], onupdate='RESTRICT', ondelete='RESTRICT',
+        ),
+        sa.CheckConstraint(
+            '0 <= estimated_prep_duration AND estimated_prep_duration <= 2400',
+            name='realistic_estimated_prep_duration',
+        ),
+    )
+
+    # Relationships
+    address = orm.relationship('Address', back_populates='restaurant')
+    orders = orm.relationship('Order', back_populates='restaurant')
+
+    def __repr__(self) -> str:
+        """Non-literal text representation."""
+        return '<{cls}({name})>'.format(cls=self.__class__.__name__, name=self.name)