Load model one to many relation eloquent way without primary key but on multiple overlapping fields

I'm working on an older project that I've been tasked to speed up certain parts of while we work on a complete re-write since the code is just badly maintained, poorly written and outdated for what it's suppose to do.

I stumbled into an issue to the core of the project and because of this I can't change it without breaking almost everything else. So I need to load a "relation" the eloquent way (using Planning:with('availability') but there isn't a real foreign ID, it rather laps with multiple fields.

Would there be a way to load it all in one query with the overlapping fields rather than have it load separately creating an n+1 problem?

+--------------+-----------------+
| Planning     | Availability    |
+--------------+-----------------+
| planning_id  | availability_id |
| date         | date            |
| startHour    | startHour       |
| stopHour     | stopHour        |
| candidate_id | candidate_id    |
| section_id   | section_id      |
+--------------+-----------------+

From the above example you can see the overlapping fields are date, startHour, stopHour, candidate_id and section_id.

I tried get...attribute but that still loads with n+1, I tried including it with ->with(['availabilities']) but that doesn't work since I ask for the model and not the relation:

return Availability::where('section_id', $this->section_id)
    ->where('candidate_id', $this->candidate_id)
    ->where('planningDate', $this->planningDate)
    ->where('startHour', $this->startHour)
    ->where('stopHour', $this->stopHour)
    ->get();

via Chebli Mohamed