public function belongsToMany($related, $foreignKey = null, $otherKey = null)
{
// First, we'll need to determine the foreign key and "other key" for the
// relationship. Once we have determined the keys we'll make the query
// instances as well as the relationship instances we need for this.
$foreignKey = $foreignKey ?: $this->getForeignKey();
$instance = new $related();
$otherKey = $otherKey ?: $instance->getForeignKey();
// Now we're ready to create a new query builder for the related model and
// the relationship instances for the relation. The relations will set
// appropriate query constraint and entirely manages the hydrations.
$query = $instance->newQuery();
return new BelongsToMany($query, $this, $foreignKey, $otherKey);
}