title: update all contact lists when some user is deleted in meteor mongdb
body: <p>We are developing a simple feature of contact list (which is a simple version of friend list ). </p>
  
  <p>As suggested <a href="https://stackoverflow.com/questions/6183147/storing-friend-relationships-in-mongodb">here</a> , I store contact_ids in contacts of each user so that userA can:</p>
  
  <ol>
  <li><p>Add user (s) to his contacts by</p>
  
  <p>Meteor.users.update(Meteor.userId(), { $push : {contacts:  newId});</p></li>
  <li><p>Remove </p>
  
  <p>Meteor.users.update(Meteor.userId(), { $pull : {contacts: existingId });</p></li>
  </ol>
  
  <p>Now what to do if userB decides to delete his account? Loop through almost a million users to pull userB id from contacts </p>