is_unique issue on update

at the end I've find my issue.
the reference {id} to check against is_unique criteria must be actively passed to the $data array. I wasn't passing it into the $data array because my update function had the $id has a passed argument and it was recalled only into the update function $model->update($id,$data)
Actually the best solution (I wasn't aware of) is to simply use the save() function for both new inserts and updates, always putting the $id into the $data array, as follows in example.

PHP Code:

function saveupdate($id=null)
{
    $model = new myModel();

    $request = service('request');
    $postData = $request->getPost();

    if (isset($postData['submit']))
    {
          $data = 
          [
              'id'    => $id,
              'firstfield'=> $this->request->getVar('firstfield'),
              'secondfield' => $this->request->getVar('secondfield'),
          ];
    
          if ($model -> save($data) === false)
          {
              return redirect()->back()->withInput()->with('errors',$model->errors());
          }
              else
          {
              return redirect()->back();
          }
    }
} 


while in the model I'll have the following validation rules:

PHP Code:

protected $validationRules = 
[
    'firstfield'    => 'required|is_unique[table.firstfield,id,{id}]',
    'secondfield'    => 'required|is_unique[table.secondfield,id,{id}]',    
];