public function newPivot(Model $parent, array $attributes, $table, $exists)
{
    return new YourCustomPivot($parent, $attributes, $table, $exists);
}

class Base extends Eloquent {

public function newPivot(Eloquent $parent, array $attributes, $table, $exists)
{
     return new Membership($parent, $attributes, $table, $exists);
}

资料说这样玩，但是还是晕，各种报错