When retrieving the related screenshot for a `Save` or `State`, we were
retrieving a very heavy representation of the associated `Rom` object,
only to iterate through its screenshots to find the one we needed.
This change modifies the `Save` and `State` models to directly query the
`Screenshot` model, which is much faster and more efficient. The
`DBScreenshotsHandler` has been updated to include a new `filter` method
that will simplify building queries using SQLAlchemy, something we can
extend to other handlers in the future.
Fixes#1925.
The current implementation for some of the database handlers, where the
same method is used to retrieve either a single entity (when an `id` is
passed), a list of entities, or `None`, makes the typing and overall
design more complex.
This change simplifies database handlers, by having two separate methods
where appropiate:
* A method that receives an `id`, and returns either an entity, or `None`.
* A method that optionally receives filters, and returns (depending on
the current handler implementation) a list of entities, or a `Select`
object that allows chaining more SQLAlchemy operations.
This change applies the guided migration process recommended by
SQLAlchemy [1], up to step 4, to have declarative ORM models that better
support Python typing.
The change was tested by running `alembic check`, which does not find
any schema changes.
Errors reported by `mypy` go down to 170, from the original 223 in the
current `master` commit.
[1] https://docs.sqlalchemy.org/en/20/changelog/whatsnew_20.html#migrating-an-existing-mapping
