Sorting Laravel model by relationship count (see comments for usage)

BaseModel.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\Relations\BelongsToMany;
use Illuminate\Database\Eloquent\Relations\HasMany;

/**
 * Class BaseModel
 * @package App
 */
abstract class BaseModel extends Model
{
    /**
     * @param $query
     * @param $relationName
     * @param string $alias
     */
    public function scopeIncludeCount($query, $relationName, $alias = null)
    {
        $alias = $alias ?: $relationName . 'Count';

        $relation = call_user_func([$this, $relationName]);

        /**
         * @var HasMany|BelongsToMany $relation
         */
        $relation->getRelated()->getTable();
        $related = $relation->getRelated();

        if ($relation instanceof HasMany) {
            $relationTable = $related->getTable();
            $query
                ->leftJoin($relationTable, $relation->getForeignKey(), '=', $relation->getQualifiedParentKeyName())
                ->groupBy($this->getTable() . '.' . $this->primaryKey)
                ->addSelect([
                    $this->getTable() . '.*',
                    \DB::raw(sprintf('COUNT(DISTINCT %s.%s) as %s', $related->getTable(), $related->primaryKey, $alias))]
                );
        } else {
            $relationTable = $relation->getTable();
            $query
                ->leftJoin($relationTable, $relation->getForeignKey(), '=', $relation->getQualifiedParentKeyName())
                ->groupBy($relation->getQualifiedParentKeyName())
                ->addSelect([
                        $this->getTable() . '.*',
                        \DB::raw(sprintf('COUNT(DISTINCT %s) as %s', $relation->getOtherKey(), $alias))
                    ]
                );
        }

    }
}

Above scope allows you to sort your models (in database) by relationship count. It works by performing a JOIN on the related model.

class User extends BaseModel {
    public function posts() {
        return $this->hasMany(Post::class);
    }
}

$query= User::query();
$query->includeCount('posts', 'postCount');
$query->orderBy('postCount', 'DESC');

$users = $query->get();

// On $user you can also do $user->postCount;