leftJoin not working with Category Model using NestableTrait

[cse111]

I have the following code in Category Model:

<?php

use Nestable\NestableTrait;

class Category extends \Eloquent {

    use NestableTrait;

    protected $parent = 'parent_id';

}

And

Category::attr(['name' => 'categories'])
    ->selected(2)
    ->renderAsDropdown();

The above code works fine. But in some other function in Controller when I use leftJoin function with Category Model, leftJoin is not working. I have something like below:

Category: leftjoin('expense','category.id','expense.category_id')->get();

in above eloquent leftjoin is not rendering in response.
Any help?