beforeInsert Validation for unique data

Hi.

I want to verify again before saving the content in the model file.

I want the category name on the form to check if it is in the same sector.

There may be a category with the same name in different sectors. So I can't set is_unique directly.

How can I add the custom error message as well?

PHP Code:

    protected $validationRules = [
        'sector_id' => 'required',
        'category' => 'required'
    ];
    
    protected $validationMessages = [
        'sector_id' => [
            'required' => 'Select sector',
        ],
        'category' => [
            'required' => 'Select '
        ]
    ];

    protected $beforeInsert = ['checkCategory'];
    
    protected $beforeUpdate = ['checkCategory'];
    
    protected function checkCategory(array $data)
    {

        $check_it = $this->where('category', $data['data']['category'])
                         ->where('sector_id', $data['data']['sector_id'])
                         ->first();

        if (!empty($check_it)) {

            $fieldName = 'category';
            $fieldRules = 'is_unique[categories.category]';
            
            $this->setValidationRule($fieldName, $fieldRules);

        }
        
        return $data;
    }