title: Store _Id as object or string in MongoDB?
body: <p>I am developing an API using Codeigniter and MongoDB.
  In this system I am saving the full name and _ID of users that the selected user
  is following.</p>
  
  <p>What is best to do regarding the _Id? Store it as an object or as a string?
  If I store it as an object I need to convert it to string when echoing out followers otherwise
  the output looks strange. </p>
  
  <p>My question is really. Is it ok to store the _Id as a string rather than an object?
  What is the downside of storing as string?</p>
  
  <p>Thankful for all input!</p>