Add CLI script to gridify all cities

- reorganize `urban_meal_delivery.console` into a sub-package
- move `tests.db.conftest` fixtures into `tests.conftest`
  => some integration tests regarding CLI scripts need a database
- add `urban_meal_delivery.console.decorators.db_revision` decorator
  to ensure the database is at a certain state before a CLI script runs
- refactor the `urban_meal_delivery.db.grids.Grid.gridify()` constructor:
  - bug fix: even empty `Pixel`s end up in the database temporarily
    => create `Pixel` objects only if an `Address` is to be assigned
       to it
  - streamline code and docstring
  - add further test cases

tests/conftest.py

@@ -1,8 +1,20 @@
-"""Utils for testing the entire package."""
+"""Fixtures for testing the entire package.
+
+The ORM related fixtures are placed here too as some integration tests
+in the CLI layer need access to the database.
+"""
 
 import os
 
+import pytest
+import sqlalchemy as sa
+from alembic import command as migrations_cmd
+from alembic import config as migrations_config
+from sqlalchemy import orm
+
+from tests.db import fake_data
 from urban_meal_delivery import config
+from urban_meal_delivery import db
 
 
 # The TESTING environment variable is set
@@ -12,3 +24,93 @@ if not os.getenv('TESTING'):
 
 if not config.TESTING:
     raise RuntimeError('The testing configuration was not loaded')
+
+
+@pytest.fixture(scope='session', params=['all_at_once', 'sequentially'])
+def db_connection(request):
+    """Create all tables given the ORM models.
+
+    The tables are put into a distinct PostgreSQL schema
+    that is removed after all tests are over.
+
+    The database connection used to do that is yielded.
+
+    There are two modes for this fixture:
+
+    - "all_at_once": build up the tables all at once with MetaData.create_all()
+    - "sequentially": build up the tables sequentially with `alembic upgrade head`
+
+    This ensures that Alembic's migration files are consistent.
+    """
+    # We need a fresh database connection for each of the two `params`.
+    # Otherwise, the first test of the parameter run second will fail.
+    engine = sa.create_engine(config.DATABASE_URI)
+    connection = engine.connect()
+
+    # Monkey patch the package's global `engine` and `connection` objects,
+    # just in case if it is used somewhere in the code base.
+    db.engine = engine
+    db.connection = connection
+
+    if request.param == 'all_at_once':
+        connection.execute(f'CREATE SCHEMA {config.CLEAN_SCHEMA};')
+        db.Base.metadata.create_all(connection)
+    else:
+        cfg = migrations_config.Config('alembic.ini')
+        migrations_cmd.upgrade(cfg, 'head')
+
+    try:
+        yield connection
+
+    finally:
+        connection.execute(f'DROP SCHEMA {config.CLEAN_SCHEMA} CASCADE;')
+
+        if request.param == 'sequentially':
+            tmp_alembic_version = f'{config.ALEMBIC_TABLE}_{config.CLEAN_SCHEMA}'
+            connection.execute(
+                f'DROP TABLE {config.ALEMBIC_TABLE_SCHEMA}.{tmp_alembic_version};',
+            )
+
+        connection.close()
+
+
+@pytest.fixture
+def db_session(db_connection):
+    """A SQLAlchemy session that rolls back everything after a test case."""
+    # Begin the outermost transaction
+    # that is rolled back at the end of each test case.
+    transaction = db_connection.begin()
+
+    # Create a session bound to the same connection as the `transaction`.
+    # Using any other session would not result in the roll back.
+    session = orm.sessionmaker()(bind=db_connection)
+
+    # Monkey patch the package's global `session` object,
+    # which is used heavily in the code base.
+    db.session = session
+
+    try:
+        yield session
+
+    finally:
+        session.close()
+        transaction.rollback()
+
+
+# Import the fixtures from the `fake_data` sub-package.
+
+make_address = fake_data.make_address
+make_courier = fake_data.make_courier
+make_customer = fake_data.make_customer
+make_order = fake_data.make_order
+make_restaurant = fake_data.make_restaurant
+
+address = fake_data.address
+city = fake_data.city
+city_data = fake_data.city_data
+courier = fake_data.courier
+customer = fake_data.customer
+order = fake_data.order
+restaurant = fake_data.restaurant
+grid = fake_data.grid
+pixel = fake_data.pixel