- the purpose of this constructor method is to generate all `Pixel`s
for a `Grid` that have at least one `Address` assigned to them
- fix missing `UniqueConstraint` in `Grid` class => it was not possible
to create two `Grid`s with the same `.side_length` in different cities
- change the `City.viewport` property into two separate `City.southwest`
and `City.northeast` properties; also add `City.total_x` and
`City.total_y` properties for convenience
- add Grid, Pixel, and AddressPixelAssociation ORM models
- each Grid belongs to a City an is characterized by the side_length
of all the square Pixels contained in it
- Pixels aggregate Addresses => many-to-many relationship (that is
modeled with SQLAlchemy's Association Pattern to implement a couple
of constraints)
- use Alembic to migrate the PostgreSQL database
+ create initial migration script to set up the database,
as an alternative to db.Base.metadata.create_all()
+ integrate Alembic into the test suite; the db_engine fixture
now has two modes:
* create the latest version of tables all at once
* invoke `alembic upgrade head`
=> the "e2e" tests are all run twice, once in each mode; this
ensures that the migration scripts re-create the same database
schema as db.Base.metadata.create_all() would
* in both modes, a temporary PostgreSQL schema is used to create the
tables in
=> could now run "e2e" tests against production database and still
have isolation
- make the configuration module public (to be used by Alembic)
- adjust linting rules for Alembic
