This PR introduces the ability to load relationship counts on an Eloquent\Collection. With the query builder you can $query->with($relations) and also $query->withCount($relations), but on the eloquent collection we can only $collection->load($relations). This introduces the ability to $collection->loadCount($relations).

Usually, of course, you would want to be loading these counts when you are retrieving the models from the DB in the first place, i.e. with the query builder as shown above, however in certain circumstances that is not possible. One example is when the model you are retrieving has a polymorphic relation on which you need to dynamically load counts based on the class type.

A real world example of how I would use this in my current app...

I am tracking certain events. An event is something like "Post created" or "Comment created". Each event has an eventable relation that is polymorphic, i.e. it might be a Post or a Comment

class Event extends Eloquent
{
    public function eventable()
    {
        return $this->morphTo();
    }
}

On my events endpoint, I want to show each event and some information about the eventable. The information shown for each eventable item can vary. For a Post it might be the number of comments it has recieved and for a Comment it might be the number of hearts it has received.

Show me the code!

$events = Event::latest()->with('eventable')->paginate();

$groups = $events->map(function ($event) {
    return $event->eventable;
})->groupBy(function ($eventable) {
    return get_class($eventable);
});

$groups[Post::class]->loadCount('comments');
$groups[Comment::class]->loadCount('hearts');

return new EventIndexResponse($events);

Not sure if this was the best example of its use, but it is a real one. Hopefully it would be useful in other situations also.

Under the hood the query is having each called on it, causing the db hits to be chunked. Only the model primary key and relation counts are being retrieved from the DB to keep the payloads and memory impact as low as possible. It is defaulting the the 1000 chunk count.

Some places where this idea has been discussed / requested:

• Ability to load count relations on Illuminate\Database\Eloquent\Collection #17845
• [Database] loadCount ideas#110

If a few people could do a once over of the test I've introduced that would be awesome - not used to mocking soooo many things 😅

Also, if there is anyway to optimise the way I'm doing this, I'm all ears 👂