title: Mongodb inserting doc without _id field
body: <p>I am newbie to mongodb.</p>
  
  <ol>
  <li><p>I need to insert a doc without the _id field generating automatically.</p></li>
  <li><p>I need to set the field Tenant_id as unique or need to change the "_id" field to Tenant_id.</p></li>
  </ol>
  
  <p>how to do it?</p>
  
  <p>something like this</p>
  
  <pre><code>     Tenant
          {Tenant_id: 123, Tenant_info: ...}
  </code></pre>