Issue in Validation when updating the record : Laravel 5.2

My rule is like below. Please check the composite_unique in the below code

public function rules()
{
    return [
      'SubCategory' => 'required|max:25|min:5|composite_unique:tblsubcategory,CategoryID',
      'CategoryID'  => 'required|min:1'
    ];
}

My update method is like below...

public function update(SubCategoryRequest $request)
{
    $SubCat = $this->CacheCollection->getAllSubCategories($request->input('CategoryID'));
    $SubCategory = $SubCat->SubCategories->where('SubCategoryID', 1)->first();
    $SubCategory->SubCategory = $request->input('SubCategory');
    $SubCategory->CategoryID = $request->input('CategoryID');
    $SubCategory->save();
}

My Validator class is like below. Please check the composite_unique rule in below code.

class ValidationServiceProvider extends ServiceProvider
{
    public function boot() {
        $this->app['validator']->extend('composite_unique', 
                                  function ($attribute, $value, $parameters, $validator) {
            // remove first parameter and assume it is the table name
            $table = array_shift( $parameters ); 

            // start building the conditions
            $fields = [ $attribute => $value ];                 

            while ( $field = array_shift( $parameters ) ) {
                $fields[ $field ] = \Request::get( $field );
            }

            // query the table with all the conditions
            $result = \DB::table( $table )
                         ->select( \DB::raw( 1 ) )
                         ->where( $fields )->first();

            return empty( $result ); // edited here
        });
    }
}

What's the problem

When i try to update the record and don't edit anything and click on update button... I am getting the error that duplicate combination of SubCategory and CategoryID. I think validation code is done just for check before Inserting the new record. For update it is not working.

Below is the Schema of SubCategory Table

CREATE TABLE `tblsubcategory` (
  `SubCategoryID` int(11) NOT NULL,
  `SubCategory` varchar(25) NOT NULL,
  `CategoryID` int(11) NOT NULL,
  `IsActive` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

ALTER TABLE `tblsubcategory`
  MODIFY `SubCategoryID` int(11) NOT NULL AUTO_INCREMENT;

Below is Unique Key Constraint

ALTER TABLE `tblsubcategory`
  ADD PRIMARY KEY (`SubCategoryID`),
  ADD UNIQUE KEY `UK_tblSubCategory_SubCategory_CategoryID` (`CategoryID`,`SubCategory`);

via Chebli Mohamed