title: Meteor: How to check if item in array field, but exclude that field in Publish?
body: <p>I'm trying to figure out how to do this in a Meteor Publish for my Posts collection:</p>
  
  <ol>
  <li>Check if the sharedBy array contains the current userId. 
  <ul>
  <li>sharedBy is an array of all the users that shared this post.</li>
  </ul></li>
  <li>If so, update the Posts collection with a boolean i.e. isSharedCurrentUser to 1.
  <ul>
  <li>preferably only the client collection gets this new field added.</li>
  </ul></li>
  <li>Publish the Posts collection but EXCLUDE the sharedBy array (since it could contain millions of entries).</li>
  </ol>