Eloquent recursive relationships

example.php

<?php

// Default setup of parent/child relationship for a hierarchichal category structure
// This prevents queries when iterating over them recursively
$categories = ParentChildHierarchy::apply(Category::all())

// Customized example where threaded comments have a custom relationship and key names
// $post->threadedCmments returns _all_ comments in a given post. Not just the top-level ones.
$comments = ParentChildHierarchy::apply($post->threadedCmments, "id", "parent_comment_id", "parentComment", "children");

ParentChildHierarchy.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Collection;

class ParentChildHierarchy
{
    public static function apply($entities, $primaryKey = "id", $parentKey = "parent_id", $parentRelation = "parent", $childRelation = "children")
    {
        $parents = $entities->keyBy($primaryKey);
        $children = $entities->groupBy($parentKey);

        // This is a manual eagerloading setup
        // that handles recursive relationships
        return $entities->map(function ($entity) use ($parents, $children, $primaryKey, $parentKey, $parentRelation, $childRelation) {
            if ($entity->$parentKey) {
                $entity->setRelation($parentRelation, $parents[$entity->$parentKey]);
            }

            $entity->setRelation($childRelation, $children[$entity->$primaryKey] ?? new Collection());

            return $entity;
        });
    }
}