title: Pass value to Json schema Definition
body: <p>I'm using AJV Schema validator on NodeJS and wish to add <code>userID</code> from <code>sessionObject</code> to every incoming payload so that I can store <code>userID</code> for each transaction. </p>
  
  <p>I wish to know if that can be done in json schemas.</p>
  
  <p>sample Incoming Client payload - </p>
  
  <pre><code>Client: {
  Client_Id: 12,
  ClientName: 'jon',
  Address: [{
  Address_Id: 22,
  Street: 'Sesimi Street',
  City: 'Mumbai'
  }
  ],
  Contact: [{
  Contact_Id: 23,
  Phone: 11111111,
  Email: "jon@doe.com"}]
  </code></pre>
  
  <p>Desired object post schema Validation - </p>
  
  <pre><code>    Client: {
          Client_Id: 12,
          ClientName: 'jon',
          UserId: 12121,
          Address: [{
          Address_Id: 22,
          Street: 'Sesimi Street',
          City: 'Mumbai',
          UserId: 12121
          }
          ],
          Contact: [{
          Contact_Id: 23,
          Phone: 11111111,
          Email: "jon@doe.com",
          UserId: 12121
  }]
  </code></pre>
  
  <p>Since incoming payloads are huge, I believe it would be best to do assignments at schema validation point instead of doing recursive operations inside of my application. I'm open to suggestions.
  Thank you</p>