title: Multiple join conditions using the $lookup operator
body: <p>Have the two following collections:</p>
  
  <pre><code>// collection1:
  {
    user1: 1,
    user2: 2,
    percent: 0.56
  }
  
  // collection2:
  {
    user1: 1,
    user2: 2,
    percent: 0.3
  }
  </code></pre>
  
  <p>I want to join these two collections on <code>user1</code> and <code>user2</code>.</p>
  
  <p>How can I write a pipeline in order to get a result like this:</p>
  
  <pre><code>{
    user1: 1,
    user2: 2,
    percent1: 0.56,
    percent2: 0.3
  }
  </code></pre>