title: Mongo does not read from primary when secondary is down
body: <p>my mongo url config is <code>url = 'mongodb://1.1.1.1:27017,2.2.2.2:27017/test';</code></p>
  
  <p>where 1.1.1.1 is primary and 2.2.2.2 is secondary. </p>
  
  <p>Initially it reads from the secondary but when I shut down secondary mongo does not read from primary and fails </p>
  
  <p>I want to know how can I read from primary when secondary is down</p>
  
  <p>I am using mongoose.</p>