Friendship system with Laravel : Many to Many relationship

tldr; you need 2 inverted relationships to make it work, check SETUP and USAGE below

---

First off the error - this is how your relation should look like:

function friends()
{
  return $this->belongsToMany('User', 'friends', 'user_id', 'friend_id')
    // if you want to rely on accepted field, then add this:
    ->wherePivot('accepted', '=', 1);
}

Then it will work without errors:

$user->friends; // collection of User models, returns the same as:
$user->friends()->get();

---

SETUP

However you would like the relation to work in both ways. Eloquent doesn't provide a relation of that kind, so you can instead use 2 inverted relationships and merge the results:

// friendship that I started
function friendsOfMine()
{
  return $this->belongsToMany('User', 'friends', 'user_id', 'friend_id')
     ->wherePivot('accepted', '=', 1) // to filter only accepted
     ->withPivot('accepted'); // or to fetch accepted value
}

// friendship that I was invited to 
function friendOf()
{
  return $this->belongsToMany('User', 'friends', 'friend_id', 'user_id')
     ->wherePivot('accepted', '=', 1)
     ->withPivot('accepted');
}

// accessor allowing you call $user->friends
public function getFriendsAttribute()
{
    if ( ! array_key_exists('friends', $this->relations)) $this->loadFriends();

    return $this->getRelation('friends');
}

protected function loadFriends()
{
    if ( ! array_key_exists('friends', $this->relations))
    {
        $friends = $this->mergeFriends();

        $this->setRelation('friends', $friends);
    }
}

protected function mergeFriends()
{
    return $this->friendsOfMine->merge($this->friendOf);
}

USAGE

With such setup you can do this:

// access all friends
$user->friends; // collection of unique User model instances

// access friends a user invited
$user->friendsOfMine; // collection

// access friends that a user was invited by
$user->friendOf; // collection

// and eager load all friends with 2 queries
$usersWithFriends = User::with('friendsOfMine', 'friendOf')->get();

// then
$users->first()->friends; // collection

// Check the accepted value:
$user->friends->first()->pivot->accepted;

It's oviously a problem in your DB and also definition of the relation. Many-to-Many relation type expects you to use and intermediate table. Here's what you have to do :

1. Create a user_friend (id, user_id, friend_id) table in your schema.
2. Remove unnecessary fields from user and friend tables.
3. Create proper foreign keys . user.id-> user_friend.user_id , friend.id -> user_friend.friend_id
4. Better define full relation on the User and Friend models,

for example :

 class User extends Eloquent {
    function friends()
    {
        return $this->belongsToMany('User', 'user_friend', 'user_id', 'friend_id');
    }
}

You can read much more in Laravel docs, HERE