title: TTL expiration of Mongoose referenced document
body: <p>I have a requirement for setting a TTL on a collection of Docs in my app but also need those docs to be referenced by another collection and am unsure how this would work? </p>
  
  <p>Does setting a TTL on a document remove that document from all other reference docs when it is expired or would I have to write a backend process myself and not use mongoDb's TTL indexes?</p>
  
  <p>An example schema of the functionality I would want :</p>
  
  <pre><code>var mongoose = require('mongoose')
    , Schema = mongoose.Schema
  
  var personSchema = Schema({
      _id     : Number,
      name    : String,
      age     : Number,
      stories : [{ type: Schema.Types.ObjectId, ref: 'Story' }]
  });
  
  var storySchema = Schema({
      _creator : { type: Number, ref: 'Person' },
      createdAt: { type: Date, expires: 60*60*24 },
      title    : String,
      fans     : [{ type: Number, ref: 'Person' }]
  });
  
  var Story  = mongoose.model('Story', storySchema);
  var Person = mongoose.model('Person', personSchema);
  </code></pre>
  
  <p>So after a Day the Story doc would expire and i would need the ref to that story to expire from the person</p>