title: Are MongoDB document IDs unique to the entire database or just that collection?
body: <p>I'm using MLab to host a MongoDB database for a node.js project I'm working on. I want to use the uids automatically given to document to identify specific documents across collections. I was wondering if the default ids are unique only in the collection the document is in, or if they are unique throughout the entire database.</p>
  
  <pre><code>{
      "_id": { // &lt;&lt;&lt;&lt; this is what I'm talking about
          "$oid": "5c0754d8d0d1c81b400690a3"
      },
      "owned": [],
      "equipped": [],
      "prospects": [],
      "username": "Urist",
      "password": "dcddb75469b4b4875094e14561e573d8",
      "date": {
          "$date": "2018-12-05T04:32:24.690Z"
      },
      "trades": [],
      "__v": 0
  }
  </code></pre>