title: MongoDB - Multiple $or operations
body: <p>How would I have multiple $or operations? So far I've tried the following but it silently ignores the 2nd $or. </p>
  
  <pre><code>{
    $or: [{a: 2}, {a: 3}], 
    $or: [{b: 5}, {b: 4}]
  }
  </code></pre>
  
  <p>I assume this is because I'm using two identical keys. Is there any way around this?</p>