Does Eloquent relation sync also remove?

问题

When updating a model and I sync a relationship, if I don't pass in all the ids that already exist, will that relationship be removed?

回答1:

You decide: sync has 2nd parameter that defaults to true and is responsible for detaching:

$model->relationship()->sync([1,2,3]);

$model->relationship()->sync([4,5,6]); // attached [4,5,6], detached [1,2,3]
$model->relationship()->getRelatedIds(); // [4,5,6]

// but:
$model->relationship()->sync([4,5,6], false); // attached [4,5,6], detached []
$model->relationship()->getRelatedIds(); // [1,2,3,4,5,6]

回答2:

The answer is Yes it does. I could not find any documentation that in fact stated that.

lets say you have 2 tables: "authors" and "books", with a pivot table "book_authors".

when creating a new author:

$author_id =2;
$author->books()->sync(array(1,4,5,15));

Now you have a 4 entries in that pivot table "book_authors":

author_id  book_id
2          1
2          4
2          5
2          15

Now update:

$author_id =2;
$author->books()->sync(array(1,15));

now "book_authors" is:

author_id  book_id
    2          1
    2          15

来源：https://stackoverflow.com/questions/24208073/does-eloquent-relation-sync-also-remove