private function belongsToMany()
    {
        // Return if there are no relationships of that type
        if (!isset($this->relationships['belongsToMany']) || empty($this->relationships['belongsToMany'])) {
            return $this;
        }

        // Group by primary key
        $this->db()->groupBy($this->table . '.' . $this->primaryKey);

        // Iterate relationships
        foreach ($this->relationships['belongsToMany'] as $join) {

            // Perform join
            $this->db()->join($join['pivot'], $this->primaryKey, '=', $join['localKey']);

            // Add related columns to select
            $column = 'GROUP_CONCAT(' . $join['pivot'] . '.' . $join['foreignKey'] . ') as concat_' . $join['alias'];
            $this->db()->addSelect($column);
        }

        // Return model instance
        return $this;
    }