title: Mongoose sort by child value
body: <p>I have a parent and child schema that looks like:</p>
  
  <pre><code>schedule = Schema ({
      from: Date,
      to: Date
  });
  
  content = Schema ({
      schedule: { type: Schema.ObjectId, ref: "schedule" }
      name: String
  });
  </code></pre>
  
  <p>My question is, how do I query Mongoose to return "all content, sorted by schedule.from date"?</p>