title: $lookup for each element within a sub-array
body: <p>So in my Database, I have the 3 collections, and they look like this: </p>
  
  <p><strong>Customers:</strong></p>
  
  <pre><code>customers = [
    {_id: 1, username: "jack", ... },
    {_id: 2, username: "jane", ... }
    ...
  ]
  </code></pre>
  
  <p><strong>Reviews:</strong></p>
  
  <pre><code>reviews = [
    { _id: 1, customerID: 1, message: "my message", ...}
    ...
  ]
  </code></pre>
  
  <p><strong>Comments:</strong></p>
  
  <pre><code>comments = [
    { _id: 1, reviewID: 1, customerID: 2, message: "my response" ...}
    ...
  ]
  </code></pre>
  
  <p>Customers can post reviews, and can also comment on other reviews. 
  So, what I want is a mongodb <code>aggregation</code> query to:</p>
  
  <ol>
  <li><p>Retrieve the reviews.</p></li>
  <li><p>The data of the customer who made the review.</p></li>
  <li><p>The comments on that review.</p></li>
  <li><p>The data of the customers who made the comment on that review.</p></li>
  </ol>
  
  <p><strong>i.e</strong></p>
  
  <pre><code>reviews = [
    {
      _id: 1,
      username: "jack",
      message: "my message"
      comments: [
        { _id: 1, username: "jane", message: "my response", ...},
        ...
      ]
      ...
    }
    ...
  ]
  </code></pre>