title: Move an array element from one array position to another (mongo)
body: <p>I have a list like this:</p>
  
  <pre><code>{ 
      arr: [
          { _id: 'a', val: 1 }, 
          { _id: 'b', val: 4 }, 
          { _id: 'd', val: 0 }, 
          { _id: 'c', val: 8 }
      ] 
  }
  </code></pre>
  
  <p>And I need to put the element <code>{_id: c}</code> before the element <code>{_id: d}</code>. How can I do it with mongo? Of course, I can download the doc to my server, prepare it and upload the one to the mongo server. But I think, it's not a good way.</p>
  
  <p><strong>P.S: sorry, I need to move the element (push it to a new position and remove the one from old position)</strong></p>