title: Unique field in react-admin
body: <p>I use <code>No</code> field as the order is very important for my list items. The problem is how I validate <code>No</code> values if it is a fresh value.</p>
  
  <pre><code>Previous list - [{No: 1, ...}, {No: 2, ...}] 
  Create new item with No = 1 should be warned.
  </code></pre>
  
  <p>Please give me any ideas to solve this problem.</p>