How to eager load a relation with a condition in Eloquent? - laravel

Question

I have a relation that can be inherited from a parent if not set for the object itself.
For an example setup let's say we have events that have a venue.
class Event extends Model
{
public function venue()
{
return $this->belongsTo('App\Venue');
}
public function activities()
{
return $this->hasMany('App\Activity');
}
}
And there are activities in the events that mostly take place in the same venue, but sometimes could be elsewhere while still belonging to the same event.
class Activity extends Model
{
public function event()
{
return $this->belongsTo('App\Event');
}
public function venue()
{
if ($this->venue_id)
return $this->belongsTo('App\Venue');
return $this->event->venue();
}
}
If I simply request activities for an event and work with them it is fine. But if I try to eager load the venues for activities, I only get the ones that are set directly on the activity, never requesting one from parent.
$activities = $event->activities;
$activities->load('venue'); // Works correctly without this line
foreach ($activities as $activity)
if ($activity->venue) // Doesn't take venue from the parent (event)
echo $activity->venue->name; //Only shows if venue_id is set on activity
Is there any chance to fix the relations so I could load them in bulk?

Answer 1

By their very nature, eager loaded relationships do not have the relationship method run for each parent model. If they did, you would have the N+1 issue, which is exactly what eager loading is meant to solve.
The relationship method is run once on the model that is used to start the query. This gets the base query to run, and then all of the parent model ids are injected into that query.
In order to do what you want, you need to change things up a little bit. First, your Activity can be directly related to venues, so setup that relationship without any conditions. Next, create an accessor method that will return the proper venue for the Activity.
So, your code would look something like:
class Activity extends Model
{
public function event()
{
return $this->belongsTo('App\Event');
}
public function venue()
{
return $this->belongsTo('App\Venue');
}
public function getActivityVenueAttribute()
{
return $this->venue ?? $this->event->venue ?? null;
}
}
The other option would be to always assign the venue_id on the Activity, even if it is the same as the Event venue_id. Then you don't need to worry about the venue id missing on the activity.