"Call to a member function active_pest() on array" when using SyncWithoutDetaching() to update a pivot table - eloquent

Question

I'm trying to using SyncWithoutDetaching() to update a pivot table (active_pest) but am getting a "Call to a member function active_pest() on array" error message.
The background here is that I have 8 tables: products, pests, actives (i.e active ingredients), crops, active_product, pest_product, crop_product, and active_pest. My form collects information about a selected (agrichemical) product - in that form, the user selects the pests, actives, and crops associated with that product. When submitted, my existing code is saving the expected information in the products table and, through a set of "belongsToMany" relationships, the active_product, pest_product, and crop_product pivot tables are also correctly updated.
For any given product there are typically 1 or 2 actives and 3-8 pests and it is those id values that I want to add to the active_pest pivot table.
My code is:
// ProductController
public function update(UpdateRequest $request)
{
$actives = $request->get('active');
$actives->active_pest()->SyncWithoutDetaching( $request->get('pest'));
...
}
// pest model
public function active_pest()
{
return $this->belongsToMany('App\Models\Pest', 'active_pest', 'active_id', 'pest_id');
}
Answers to other questions about this type of error message indicate that there is something wrong with the active_pest() relationship - but I got the same error message after making a typo there (active_pestr). Regardless, I don't understand what I'm doing wrong.
Insight appreciated. Thanks, Tom

Answer 1

$actives = $request->get('active');
Just return array, so you are trying something like this [1,2,3]->active_pest().
Which is exactly what it says. Call to a member function active_pest() on array.
You need to work with eloquent instance to perform ->active_pest()->SyncWithoutDetaching( $request->get('pest'));
So you can find you instances like this:
$actives = Active::whereIn('col_name', $request->get('active'))->get();
This will return you collection of instances, to save pest for each of them you need to iterate with foreach like this:
foreach($actives as $active){
$active->active_pest()->SyncWithoutDetaching( $request->get('pest'));
}