title: Mongoose pre save is not running with discriminators
body: <p>I am trying to call pre save hook before saving owner in mongoose. Pre save hook is not called. Is there any way to do it ?</p>
  
  <pre><code>const baseOptions = {
      discriminatorKey: '__type',
      collection: 'users'
  }
  const Base = mongoose.model('Base', new mongoose.Schema({}, baseOptions));
  
  const Owner = Base.discriminator('Owner', new mongoose.Schema({
      firstName: String,
      email: String,
      password: String,
  
  }));
  
  const Staff = Base.discriminator('Staff', new mongoose.Schema({
      firstName: String,     
  }));
  </code></pre>
  
  <p>this is not called</p>
  
  <pre><code> Owner.schema.pre('save', function (next) {
      if (!!this.password) {
          // ecryption of password
      } else {
          next();
      }
  })
  </code></pre>