title: Laravel validations: opposite of exists (value should not exist in the given table,column)
body: <p>I have table <code>employee</code> that has a nullable column <code>substitute</code> referencing to another employee. The API I'm developing can receive a DELETE request for <code>/employee/{employee_id}</code> and it should validate employee_id to make sure it is not referenced by any other row. So basically, I need a validation rule like this:</p>
  
  <pre><code>Validator::make($data, [
      'employee_id' =&gt; '!exists:employee,substitute'
  ], [
      '!exists' =&gt; 'Employee :employee_id cannot be deleted as it is being used as substitute for other employees' // custom message
  ])
  </code></pre>
  
  <p>Does Laravel have something similar out of the box or should I define custom validation rule?</p>