Use globals for the database connection

- remove the factory functions for creating engines and sessions
- define global engine, connection, and session objects to be used
  everywhere in the urban_meal_delivery package

src/urban_meal_delivery/configuration.py

@@ -85,7 +85,10 @@ def make_config(env: str = 'production') -> Config:
         raise ValueError("Must be either 'production' or 'testing'")
 
     # Without a PostgreSQL database the package cannot work.
-    if config.DATABASE_URI is None:
+    # As pytest sets the "TESTING" environment variable explicitly,
+    # the warning is only emitted if the code is not run by pytest.
+    # We see the bad configuration immediately as all "db" tests fail.
+    if config.DATABASE_URI is None and not os.getenv('TESTING'):
         warnings.warn('Bad configurartion: no DATABASE_URI set in the environment')
 
     return config

src/urban_meal_delivery/db/__init__.py

@@ -3,8 +3,9 @@
 from urban_meal_delivery.db.addresses import Address
 from urban_meal_delivery.db.addresses_pixels import AddressPixelAssociation
 from urban_meal_delivery.db.cities import City
-from urban_meal_delivery.db.connection import make_engine
-from urban_meal_delivery.db.connection import make_session_factory
+from urban_meal_delivery.db.connection import connection
+from urban_meal_delivery.db.connection import engine
+from urban_meal_delivery.db.connection import session
 from urban_meal_delivery.db.couriers import Courier
 from urban_meal_delivery.db.customers import Customer
 from urban_meal_delivery.db.grids import Grid

src/urban_meal_delivery/db/connection.py

@@ -1,17 +1,26 @@
-"""Provide connection utils for the ORM layer."""
+"""Provide connection utils for the ORM layer.
+
+This module defines fully configured `engine`, `connection`, and `session`
+objects to be used as globals within the `urban_meal_delivery` package.
+
+If a database is not guaranteed to be available, they are set to `None`.
+That is the case on the CI server.
+"""
+
+import os
 
 import sqlalchemy as sa
-from sqlalchemy import engine
 from sqlalchemy import orm
 
 import urban_meal_delivery
 
 
-def make_engine() -> engine.Engine:  # pragma: no cover
-    """Provide a configured Engine object."""
-    return sa.create_engine(urban_meal_delivery.config.DATABASE_URI)
+if os.getenv('TESTING'):
+    engine = None
+    connection = None
+    session = None
 
-
-def make_session_factory() -> orm.Session:  # pragma: no cover
-    """Provide a configured Session factory."""
-    return orm.sessionmaker(bind=make_engine())
+else:  # pragma: no cover
+    engine = sa.create_engine(urban_meal_delivery.config.DATABASE_URI)
+    connection = engine.connect()
+    session = orm.sessionmaker(bind=connection)()