title: Ruby on Rails: validate when context is not given value
body: <p>I update record by passing context:</p>
  
  <pre><code>employee.attributes = employee_params
  employee.save(context: :age_related_fields)
  </code></pre>
  
  <p>and I know that in model I can conditionally apply validation only for given context</p>
  
  <pre><code>validates :date_of_birth, on: :age_related_fields
  </code></pre>
  
  <p>but is there a way how to use negation in the <code>:on</code> condition?
  e.g.</p>
  
  <p>validate <code>date_of_birth</code> when context is NOT <code>:personal_fields</code>.</p>