title: Populate returns whole parent document
body: <p>I just started learning express and mongodb. I recently faced the kind of problem, I'm trying to select all the subdocuments which are inside of <code>Room</code> model.</p>
  <pre><code>const books = await Room.find().populate('book');
  </code></pre>
  <p>But it returns whole room document when i want to select only <code>bookings</code> field.</p>
  <p>Here's the book schema</p>
  <pre><code>const bookSchema = new mongoose.Schema({
    startDate: {
      type: Date,
      required: true,
    },
    endDate: {
      type: Date,
      required: true,
    },
    name: {
      type: String,
      required: true,
    },
    phone: {
      type: String,
      required: true,
    },
  });
  
  module.exports = mongoose.model(&quot;book&quot;, bookSchema)
  </code></pre>
  <p>And here's the room schema</p>
  <pre><code>const roomSchema = new mongoose.Schema({
    currentlyReserved: {
      type: Boolean,
      default: false,
    },
    people: {
      type: Number,
      required: true,
    },
    roomNumber: {
      type: Number,
      required: true,
    },
    pricePerPerson: {
      type: Number,
      required: true,
    },
    reservedUntil: {
      type: Date,
      default: null,
    },
    reservedBy: {
      type: String,
      default: null,
    },
    bookings: {
      type: [{ type: mongoose.Schema.Types.ObjectId, ref: &quot;book&quot; }],
    },
  });
  
  module.exports = mongoose.model(&quot;room&quot;, roomSchema);
  
  </code></pre>