title: Mongoose: multiple query populate in a single call
body: <p>In Mongoose, I can use a query populate to populate additional fields after a query. I can also populate multiple paths, such as</p>
  
  <pre><code>Person.find({})
   .populate('books movie', 'title pages director')
   .exec()
  </code></pre>
  
  <p>However, this would generate a lookup on book gathering the fields for title, pages and director - and also a lookup on movie gathering the fields for title, pages and director as well. What I want is to get title and pages from books only, and director from movie. I could do something like this:</p>
  
  <pre><code>Person.find({})
   .populate('books', 'title pages')
   .populate('movie', 'director')
   .exec()
  </code></pre>
  
  <p>which gives me the expected result and queries.</p>
  
  <p>But is there any way to have the behavior of the second snippet using a similar "single line" syntax like the first snippet? The reason for that, is that I want to programmatically determine the arguments for the populate function and feed it in. I cannot do that for multiple populate calls.</p>