Laravel全局作用域和连接

我在Laravel 5.1中设置了一个全局作用域，它工作得很好。然而，在我的一些页面上，我使用MySQL joins使用雄辩的建设者。这会导致一个模棱两可的错误:

Column 'cust_id' in where clause is ambiguous

我不知道如何避免这个问题。我知道我可以使用子查询代替，但有没有其他的解决方案?

这是我的作用域文件:

<?php
namespace App'Scopes;
use Illuminate'Database'Eloquent'Model;
use Illuminate'Database'Eloquent'Builder;
use Illuminate'Database'Eloquent'ScopeInterface;
use App'Customers;
use DB, Session;
class MultiTenantScope implements ScopeInterface
{
    /**
     * Create a new filter instance.
     *
     * @param  UsersRoles $roles
     * @return void
     */
    public function __construct()
    {
        $this->custId = Session::get('cust_id');
    }
    /**
     * Apply scope on the query.
     *
     * @param Builder $builder
     * @param Model $model
     * @return void
     */
    public function apply(Builder $builder, Model $model)
    {
        if($this->custId) 
        {
            $builder->whereCustId($this->custId); 
        } 
        else 
        {
            $model = $builder->getModel();
            $builder->whereNull($model->getKeyName()); 
        }
    }
    /**
     * Remove scope from the query.
     *
     * @param Builder $builder
     * @param Model $model
     * @return void
     */
    public function remove(Builder $builder, Model $model)
    {
        $query = $builder->getQuery();
        $query->wheres = collect($query->wheres)->reject(function ($where) 
        {
            return ($where['column'] == 'cust_id');
        })->values()->all();
    }  
}