title: Push value to Array if key does not exist Mongoose
body: <p>Given mongoose schema</p>
  
  <pre><code>var SomeSchema = new Schema({
      // ...
      members: [
          {
              name: String,
              username: String
          }
      ]
  });
  </code></pre>
  
  <p>From my code I want to push object to <code>members</code> but only if there is no given username in array yet. How can I do it with mongoose?</p>