title: Validate Nova on multiple unique fields
body: <p>I've created a Nova resource whose underlying model links to itself as a parent. I need a validation rule where the text is unique per parent ID.</p>
  
  <pre><code>id  text  parent_id
  1   A     NULL
  2   B     NULL
  3   C     NULL
  4   D     1
  5   E     1
  6   F     2
  </code></pre>
  
  <p>So, if I want to create a new item with <code>text:C</code>, <code>parent_id:NULL</code>, I can't because <code>id:3</code> already has <code>text:C</code>, <code>parent_id:NULL</code>.</p>
  
  <p>If I want to update the text of <code>id:5</code> to <code>D</code>, I can't because <code>id:4</code> already has <code>text:D</code>, <code>parent_id:1</code>.</p>
  
  <p>There doesn't seem to be a way to access the database from within a validation rule.</p>
  
  <p>I've tried creating a Rule, then getting the <code>route()</code>, but it returns an <code>Illuminate\Http\Request</code>, not a <code>Laravel\Nova\Http\Requests\NovaRequest</code>, from which I'd be able to get the underlying model, and then build a query.</p>
  
  <p>How can I validate complex rules such as these against the database?</p>