title: How to use mongoose model schema with dynamic keys?
body: <p>i'm using mongoose with nodejs, and i need to create a dynamic schema model,
  this is my code:</p>
  
  <pre><code>schema.add({key : String});
  </code></pre>
  
  <p>key = "user_name", but in my db i found that the model take it as key</p>
  
  <pre><code>{ key : "Michele" } and not { user_name: "Michele"}
  </code></pre>
  
  <p>What can i do? thank you.</p>