为什么Laravel / Eloquent不能使用JOIN进行预先加载？

<?php

class Cat extends Eloquent {

    public function user() {
        return $this->belongsTo('User');
    }
}

class User extends Eloquent {

    public function cats() {
        return $this->hasMany('Cat');
    }
}
<?php

class Cat extends Eloquent {

    public