Laravel save / update many to many relationship

Question

Can anyone help me on how to save many to many relationship? I have tasks, user can have many tasks and task can have many users (many to many), What I want to achieve is that in update form admin can assign multiple users to specific task. This is done through html multiple select input

name="taskParticipants[]"

The catch here is that through the same form (input) you can add/remove users, that's why I have to use sync(). Maybe I should start from the beginning but don't know where to start...

This is my User model:

public function tasks()
{
    return $this->belongsToMany('Task','user_tasks');
}

Task model

public function taskParticipants()
{
    return $this->belongsToMany('User','user_tasks');
}

TaskController

public function update($task_id)
{
    if (Input::has('taskParticipants'))
    {
        foreach(Input::get('taskParticipants') as $worker)
        {
            $task2 = $task->taskParticipants->toArray();
            $task2 = array_add($task2,$task_id,$worker);
            $task->taskParticipants()->sync(array($task2));
        }
    }
}

This is structure of tables tasks id|title|deadline

user_tasks
id|task_id|user_id

Answer 1

tldr; Use sync with 2nd param false

---

Many-to-many relationship is belongsToMany on both models:

// Task model
public function users()
{
  return $this->belongsToMany('User', 'user_tasks'); // assuming user_id and task_id as fk
}

// User model
public function tasks()
{
  return $this->belongsToMany('Task', 'user_tasks');
}

---

In order to add new relation use attach or sync.

Difference between the two is:

1 attach will add new row on the pivot table without checking if it's already there. It's good when you have additional data linked to that relation, for example:

User and Exam linked with pivot table attempts: id, user_id, exam_id, score

I suppose this is not what you need in your situation:

$user->tasks()->getRelatedIds(); // [1,2,3,4,5,6]

$user->tasks()->attach([5,6,7]);
// then
$user->tasks()->getRelatedIds(); // [1,2,3,4,5,6,5,6,7]

---

2 sync on the other hand, will either remove all relations and set them up anew:

$user->tasks()->getRelatedIds(); // [1,2,3,4,5,6]

$user->tasks()->sync([1,2,3]);
// then
$user->tasks()->getRelatedIds(); // [1,2,3]

or it will setup new relations without detaching previous AND without adding duplicates:

$user->tasks()->sync([5,6,7,8], false); // 2nd param = detach
// then
$user->tasks()->getRelatedIds(); // [1,2,3,4,5,6,7,8]