Laravel Eloquent - Attach vs Sync

What is the difference between attach() and sync() in Laravel 4\'s Eloquent ORM? I\'ve tried to look around but couldn\'t find anything!

attach():

Example:

$user = User::find(1);

$user->roles()->attach(1);

sync()

Similar to the attach() method. sync() also use to attach related models. However main difference is:

Sync method accepts an array of IDs to place on the pivot table
Secondly, most important, The sync method will delete the models from table if model does not exist in array and insert new items to the pivot table.

Example:

user_role

id  user_id role_id
1    12       1
2    12       5
3    12       2

$user = User::find(12);
$user->roles()->sync(array(1, 2, 3));

The above operation will delete:

id  user_id role_id
2    12       5

And insert role_id 3 to the table.

user_role table

id  user_id role_id
1    12       1
3    12       2
4    12       3

To make it even simpler:

The attach function only add records to the Pivot table.

The sync function replaces the current records with the new records. This is very useful for updating a model.

Example:

Assuming you have a created Post that has many Tags attached on it where the Tags ID's are [1,2,3].

And the user has the ability to update the Post and its Tags.

The user will send you the new Tags ID's [3,4,5].

If you use the sync function, the new Tags of the Post will be [3,4,5] only.

But if you use the attach function, the new Tags of the Post will be [1,2,3,4,5].

来源：https://stackoverflow.com/questions/23968415/laravel-eloquent-attach-vs-sync

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