Use SQLAlchemy relationships instead of the foreign keys directly

[mprahl]

In models.py, there are a few places where the foreign key is directly used instead of a relationship. We should use a relationship when possible.

For example:
https://github.com/release-engineering/cachito/blob/b3a7c5a256f450cd16f419dbf4e420690ed8f533/cachito/web/models.py#L540-L543

This is another instance:
https://github.com/release-engineering/cachito/blob/b3a7c5a256f450cd16f419dbf4e420690ed8f533/cachito/web/models.py#L574

Investigate if its worth using an alternate join condition here instead of this method:
https://github.com/release-engineering/cachito/blob/b3a7c5a256f450cd16f419dbf4e420690ed8f533/cachito/web/models.py#L387-L391

This might be another potential area for improvement (needs investigation):
https://github.com/release-engineering/cachito/blob/b3a7c5a256f450cd16f419dbf4e420690ed8f533/cachito/web/models.py#L312-L349

This might be also another potential area for improvement (needs investigation):
https://github.com/release-engineering/cachito/blob/b3a7c5a256f450cd16f419dbf4e420690ed8f533/cachito/web/models.py#L351-L377

[tkdchen]

@mprahl I makde some changes to the code for this issue, however I can't figure out an improvement to dependencies_count and packages_count except changing the filter criterion like RequestDependency.request_id == self.id based on a relationship property. Any idea? What is your thought on the potential area for improvement originally?

[mprahl]

@mprahl I makde some changes to the code for this issue, however I can't figure out an improvement to dependencies_count and packages_count except changing the filter criterion like RequestDependency.request_id == self.id based on a relationship property. Any idea? What is your thought on the potential area for improvement originally?

@tkdchen I think it'd be cleaner to use a relationship instead of the foreign keys directly in the filter as you mentioned, but it's fine to keep it as is. I'll leave the decision up to you if you'd like to change it.