title: Prevent field modification with Mongoose Schema
body: <p>Is there any way to set a field with an "unmodifiable" setting (Such as type, required, etc.) when you define a new Mongoose Schema? This means that once a new document is created, this field can't be changed. </p>
  
  <p>For example, something like this:</p>
  
  <pre><code>var userSchema = new mongoose.Schema({
    username: {
      type: String,
      required: true,
      unmodifiable: true
    }
  })
  </code></pre>