title: default value in MongoDB
body: <p>I was wondering if it is possible to set a default value choosing from a array of elements..
  For example:</p>
  <pre><code>    const userSchema = new mongoose.Schema({
         username: {
        type: 'String',
        require: [ true, &quot;Please choose a username!&quot; ],
        trim: true,
        unique: true
     },
     
     ...etc ...
     .....
      avatar: {
        type: 'String',
        default: '[FOR EXAMPLE HERE, TO TAKE RANDOMLY A VALUE FROM A LIST]'
     },
  </code></pre>
  <p>To be more precise, I want to design a couple of avatar logos for the users that don't select a photo for their avatars. So I want to have a list, with the picture urls. so ..it's possible to randomly choose a default value from a list?</p>