Sync function in laravel not working as expected - php

Question

I have an edit form which should update an activity with selected categories, but the sync function isn't working as expected.
It adds the selected checkboxes to the database but doesn't delete the old values. Resulting in mutliple of the same values in the pivot table.
form code:
<p class="activiteit-categorie">
<label>categorie</label>
#foreach ($categories as $categorie)
<div class="categorie-input">
<input type="checkbox" name="categories[]" value="{{ $categorie->id }}"><label for="categorie">{{$categorie->name}}</label><br>
</div>
#endforeach
</p>
controller code:
if ($request->has('categories')) {
$activiteit->categorie()->sync($request->input('categories'));
} else {
$activiteit->categorie()->detach($request->input('categories'));
}
return redirect()->route('active-overview')->with('message', 'Activiteit is succesvol aangepast!');
models categorie:
namespace App;
use Illuminate\Database\Eloquent\Model;
class categorie extends Model
{
protected $fillable = [
'name'
];
public function activiteit() {
return $this->BelongsToMany('App\activiteit');
}
}
model activiteit:
<?php
namespace App;
use Illuminate\Database\Eloquent\Model;
class activiteit extends Model
{
protected $fillable = [
'name', 'image', 'intro', 'goal', 'traject', 'extra_info', 'duration', 'price'
];
public function categorie() {
return $this->belongsToMany('App\categorie');
}
}

Answer 1

You can try this
class categorie extends Model{
protected $fillable = [
'name'
];
public function activiteit() {
return $this->BelongsToMany('App\activiteit','pivot_table_name','categorie_id','activiteit_id');
}
}
Use like this another Model
if ($request->has('categories')) {
$activiteit->categorie()->sync($request->categories);
} else {
$activiteit->categorie()->detach($request->categories);
}
It works fine for me

Answer 2

So after trying some stuff out I came up with a simple solution to my problem.
I first detach all the categories from the selected activity and then add selected categories to the pivot table.
$activiteit->categorie()->detach();
if ($request->has('categories')) {
$activiteit->categorie()->sync($request->categories);
}