laravel 5.6：多对多关系同步

Question

I create 3 tables

articles : id title

category: id name

article_category:  id article_id category_id main

And in Category model:

public function articles()
{
    return $this->belongsToMany(Article::class);
}

I want add data to article_category:

$article_id = 20;
$category_id = 1;
$category = Category::find($category_id);
$cat[$article_id] = ['main'=>false];
$category ->articles()->sync($cat);

$article_id = 20;
$category_id = 2;
$category = Category::find($category_id);
$cat[$article_id] = ['main'=>false];
$category ->articles()->sync($cat);

Now In article_category I have:

article_id    category_id   main
20              1           false
20              2           false

Ok It seems normal.

for example if add this again:

$article_id = 20;
$category_id = 2;
$category = Category::find($category_id);
$cat[$article_id] = ['main'=>false];
$category ->articles()->sync($cat);

It not generate duplication rows.

Now my problem is this:

If I add a new article_id, It remove all last saved records.

for example with add these codes:

$article_id = 21;
$category_id = 1;
$category = Category::find($category_id);
$cat[$article_id] = ['main'=>false];
$category ->articles()->sync($cat);

$article_id = 21;
$category_id = 2;
$category = Category::find($category_id);
$cat[$article_id] = ['main'=>false];
$category ->articles()->sync($cat);

I expect this:

article_id    category_id   main
20              1           false
20              2           false
21              1           false
21              2           false

But In my databse I have this:

article_id category_id main 21 1 false 21 2 false

what's my wrong?

Answer 1

As per the documentation in Laravel 5.6, Any IDs that are not in the given array will be removed from the intermediate table. It will only make many to many relation for category_id in which $cat variable contains its respective id.

This is why, it will remove all last saved records when adding a new article.

If you want to attach the new relationship, simply use

 $category->articles()->syncWithoutDetaching($cat);