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/addresses.py

@@ -0,0 +1,82 @@
+"""Provide the ORM's Address model."""
+
+import sqlalchemy as sa
+from sqlalchemy import orm
+from sqlalchemy.dialects import postgresql
+from sqlalchemy.ext import hybrid
+
+from urban_meal_delivery.db import meta
+
+
+class Address(meta.Base):
+    """An Address of a Customer or a Restaurant on the UDP."""
+
+    __tablename__ = 'addresses'
+
+    # Columns
+    id = sa.Column(sa.Integer, primary_key=True, autoincrement=False)  # noqa:WPS125
+    _primary_id = sa.Column('primary_id', sa.Integer, nullable=False, index=True)
+    created_at = sa.Column(sa.DateTime, nullable=False)
+    place_id = sa.Column(
+        sa.Unicode(length=120), nullable=False, index=True,  # noqa:WPS432
+    )
+    latitude = sa.Column(postgresql.DOUBLE_PRECISION, nullable=False)
+    longitude = sa.Column(postgresql.DOUBLE_PRECISION, nullable=False)
+    _city_id = sa.Column('city_id', sa.SmallInteger, nullable=False, index=True)
+    city_name = sa.Column('city', sa.Unicode(length=25), nullable=False)  # noqa:WPS432
+    zip_code = sa.Column(sa.Integer, nullable=False, index=True)
+    street = sa.Column(sa.Unicode(length=80), nullable=False)  # noqa:WPS432
+    floor = sa.Column(sa.SmallInteger)
+
+    # Constraints
+    __table_args__ = (
+        sa.ForeignKeyConstraint(
+            ['primary_id'], ['addresses.id'], onupdate='RESTRICT', ondelete='RESTRICT',
+        ),
+        sa.ForeignKeyConstraint(
+            ['city_id'], ['cities.id'], onupdate='RESTRICT', ondelete='RESTRICT',
+        ),
+        sa.CheckConstraint(
+            '-90 <= latitude AND latitude <= 90', name='latitude_between_90_degrees',
+        ),
+        sa.CheckConstraint(
+            '-180 <= longitude AND longitude <= 180',
+            name='longitude_between_180_degrees',
+        ),
+        sa.CheckConstraint(
+            '30000 <= zip_code AND zip_code <= 99999', name='valid_zip_code',
+        ),
+        sa.CheckConstraint('0 <= floor AND floor <= 40', name='realistic_floor'),
+    )
+
+    # Relationships
+    city = orm.relationship('City', back_populates='addresses')
+    restaurant = orm.relationship('Restaurant', back_populates='address', uselist=False)
+    orders_picked_up = orm.relationship(
+        'Order',
+        back_populates='pickup_address',
+        foreign_keys='[Order._pickup_address_id]',
+    )
+
+    orders_delivered = orm.relationship(
+        'Order',
+        back_populates='delivery_address',
+        foreign_keys='[Order._delivery_address_id]',
+    )
+
+    def __repr__(self) -> str:
+        """Non-literal text representation."""
+        return '<{cls}({street} in {city})>'.format(
+            cls=self.__class__.__name__, street=self.street, city=self.city_name,
+        )
+
+    @hybrid.hybrid_property
+    def is_primary(self) -> bool:
+        """If an Address object is the earliest one entered at its location.
+
+        Street addresses may have been entered several times with different
+        versions/spellings of the street name and/or different floors.
+
+        `is_primary` indicates the first in a group of addresses.
+        """
+        return self.id == self._primary_id