laravel call to a member function categories() on integer - php

Question

I am trying to update and existing record in pivot table for many to many relationsip with laravel.
I have store function that work :
public function store(Request $request)
{
$produk = new product($request->input()) ;
$feat = (boolean)$request->input('featured');
$input = $request->all();
$input['featured'] = $feat;
$inputproduk = product::create($input);
$inputproduk->categories()->attach($request->input('kat_id'));
return redirect()->back()->with('message', 'Memasukkan Data Produk Berhasil');
}
But why my update function didn't work???
public function update(Request $request, $id)
{
$produk = Product::FindorFail($id);
$produk = new product($request->input()) ;
$input = $request->except('_method', '_token', 'picture', 'kat_id');
$inputproduk = product::whereId($id)->update($input);
$inputproduk->categories()->sync($request->input('kat_id'));
return redirect()->back()->with('message', 'Mengedit Data Produk Berhasil');
}
The error is :
Call to a member function categories() on integer
What is that mean? Please help me.
View :
<div class="form-group">
<label for="KategoriProduk">Kategori Produk</label>
<select class="form-control" name="kat_id[]" multiple>
#foreach($kategoris as $value)
<option value="{{$value->id}}">{{$value->namekat}}</option>
#endforeach
</select>
</div>
I only post partially because if I post it all then the question is just full of code. Tell me if you need more.

Answer 1

It's because of the following line.
$inputproduk = product::whereId($id)->update($input);
When you call update it will return an integer not a product object.
What you can do is first get the product and then update.
$inputproduk = product::whereId($id)->first();
$inputproduk->update($input);
$inputproduk->categories()->sync($request->input('kat_id'));