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/cities.py

@@ -0,0 +1,83 @@
+"""Provide the ORM's City model."""
+
+from typing import Dict
+
+import sqlalchemy as sa
+from sqlalchemy import orm
+from sqlalchemy.dialects import postgresql
+
+from urban_meal_delivery.db import meta
+
+
+class City(meta.Base):
+    """A City where the UDP operates in."""
+
+    __tablename__ = 'cities'
+
+    # Generic columns
+    id = sa.Column(  # noqa:WPS125
+        sa.SmallInteger, primary_key=True, autoincrement=False,
+    )
+    name = sa.Column(sa.Unicode(length=10), nullable=False)
+    kml = sa.Column(sa.UnicodeText, nullable=False)
+
+    # Google Maps related columns
+    _center_latitude = sa.Column(
+        'center_latitude', postgresql.DOUBLE_PRECISION, nullable=False,
+    )
+    _center_longitude = sa.Column(
+        'center_longitude', postgresql.DOUBLE_PRECISION, nullable=False,
+    )
+    _northeast_latitude = sa.Column(
+        'northeast_latitude', postgresql.DOUBLE_PRECISION, nullable=False,
+    )
+    _northeast_longitude = sa.Column(
+        'northeast_longitude', postgresql.DOUBLE_PRECISION, nullable=False,
+    )
+    _southwest_latitude = sa.Column(
+        'southwest_latitude', postgresql.DOUBLE_PRECISION, nullable=False,
+    )
+    _southwest_longitude = sa.Column(
+        'southwest_longitude', postgresql.DOUBLE_PRECISION, nullable=False,
+    )
+    initial_zoom = sa.Column(sa.SmallInteger, nullable=False)
+
+    # Relationships
+    addresses = orm.relationship('Address', back_populates='city')
+
+    def __repr__(self) -> str:
+        """Non-literal text representation."""
+        return '<{cls}({name})>'.format(cls=self.__class__.__name__, name=self.name)
+
+    @property
+    def location(self) -> Dict[str, float]:
+        """GPS location of the city's center.
+
+        Example:
+            {"latitude": 48.856614, "longitude": 2.3522219}
+        """
+        return {
+            'latitude': self._center_latitude,
+            'longitude': self._center_longitude,
+        }
+
+    @property
+    def viewport(self) -> Dict[str, Dict[str, float]]:
+        """Google Maps viewport of the city.
+
+        Example:
+            {
+                'northeast': {'latitude': 48.9021449, 'longitude': 2.4699208},
+                'southwest': {'latitude': 48.815573, 'longitude': 2.225193},
+            }
+        """  # noqa:RST203
+        return {
+            'northeast': {
+                'latitude': self._northeast_latitude,
+                'longitude': self._northeast_longitude,
+            },
+            'southwest': {
+                'latitude': self._southwest_latitude,
+                'longitude': self._southwest_longitude,
+            },
+        }