title: Schema for optional conditions in Joi
body: <p>Suppose I have an object like:</p>
  
  <pre><code>{
    a : 1,
    b : 2,
    c : 3,
    d : 4
  }
  </code></pre>
  
  <p><strong>At least 1 of pair</strong> out of <code>[a,b], [a,c], [d]</code> should have validation passed(have correct values).</p>
  
  <p>Assume all values are <strong>numbers</strong>.</p>
  
  <p>How can I create Joi schema for it.</p>