title: Mongoose populate multiple nested documents
body: <p>I've searched high and low but can't figure out how to form the following populate query, first here are my models:</p>
  
  <pre><code>const CourseSchema = new Schema({
      classes: [{ type: Schema.Types.ObjectId, ref: 'Classroom' }]
  });
  
  const ClassSchema = new Schema({
      location: { type: mongoose.Schema.Types.ObjectId, ref: 'Location' },
      instructors: [{ type: mongoose.Schema.Types.ObjectId, ref: 'User' }],
  });
  </code></pre>
  
  <p>I have an endpoint which gets a single course, but I want to populate the <code>classes</code> field AND the <code>location</code> and <code>instructors</code> fields in <code>classes</code>. Right now, I can either populate the instructors field in <code>classes</code> or <code>location</code>, but I can't populate both of them at the same time. This is what I have for now:</p>
  
  <pre><code>    Course
          .findById(req.params.courseId)
          .populate({
              path: 'classes',
              populate: {
                  path: 'instructors',
                  model: 'User'
              }
          })
  </code></pre>
  
  <p>How can I also populate the <code>location</code> field in <code>classes</code>?</p>
  
  <p>Thanks.</p>