title: Mongoose. Remove fields on save that are not present in schema
body: <p>I have a doc saved in MongoDB like this:</p>
  
  <pre><code>{
    title: 'title',
    author: 'author name',
    body: 'the body',
    admin: 'admin user'
  }
  </code></pre>
  
  <p>And this is the schema in Mongoose: </p>
  
  <pre><code>var blogSchema = new Schema({
    title:  String,
    author: String,
    body:   String
  });
  </code></pre>
  
  <p>I would like that on save, the "admin" field was removed from the doc, is this possible? Can I automatize this in event <strong>pre-save</strong>?</p>
  
  <p>Thanks.</p>