Laravel：无法加入表格

I am trying to join two tabels using Laravel (just started using it). One table holds user information, and the other table holds a users friend IDs.

So one user can have multiple friends. I have done this is the user class:

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class User extends Model
{
    protected $table="user_info";

    public function getFriends()
    {
        //user_id is the column in the user_friends table that should be linked to the id table in this class table (user_info)

        return $this->hasMany('App\UserFriends','user_id','id');
    }
}

And the other class (UserFriends)

namespace App;

use Illuminate\Database\Eloquent\Model;

class UserFriends extends Model
{
    protected $table="user_friends";
}

Then in a controller I call

$t = $this->sql_obj->getFriends()->toSql();

However, if I print the result of this it returns this:

select * from `user_friends` where `user_friends`.`user_id` is null and `user_friends`.`user_id` is not null

Why is it not joining the tables?

Thanks for any help!

in Your UserModel :

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class User extends Model
{
    protected $table="user_info";

    public function getFriends()
    {
       return $this->belongsToMany('App\User', 'user_friends', 'user_id', 'id');
    }
}

in your controller :

if you want to get Auth user's friend :

 public function friends(Request $request){

      $friends = Auth::user()->getFriends;
      return response()->json(array('friends'=>$friends));   

 }

if you want to get other user's friends :

  public function friends(Request $request){
      $userid=$request->input('userid');
      $user=User::find($userid);
      $friends = $user->getFriends;
      return response()->json(array('friends'=>$friends));   
 }

or If you want to use query builder to join :

   use DB;

 public function friends(Request $request){
    $userid=$request->input('userid');  // Auth::user()->id

    $friends= DB::table('user_info')
    ->join('user_friends', 'user_info.id', '=', 'user_friends.user_id')
    ->where('user_friends.user_id', '=',$userid)         
    ->select('user_info.*')
    ->get();

    return response()->json(array('friends'=>$friends));   
 }