title: Firebase: FieldValue.arrayUnion with objects
body: <p>For example I have a doc like this:</p>
  
  <pre><code>team: {
  members: [{name: "john", age: 2}, {name: "tim", age: 3}]
  }
  </code></pre>
  
  <p>And I want to update Tims age to 4 <code>{name: "tim", age: 3}</code> to <code>{name: "tim", age: 4}</code></p>
  
  <p>Can I do this using <strong>FieldValue.arrayUnion</strong> ?</p>
  
  <p>Because right now.</p>
  
  <pre><code>...
  .update({team: firbase.firestore.FieldValue.arrayUnion({name: "tim", age: 4})})
  </code></pre>
  
  <p>Will create new entry and team will be like </p>
  
  <pre><code>[
   {name: "john", age: 2}, 
   {name: "tim", age: 3}, 
   {name: "tim", age: 4} &lt;-- duplicate
  ]
  </code></pre>