title: Arithmetic operations with fields
body: <p>Is it possible to query the result of a subtraction between two fields?</p>
  
  <p>E.g. There are two fields: "start", "end". I would like documents with <code>end - start &gt; 10</code>.</p>
  
  <p>Can this be done directly or the only way to do is to create a new field while loading the documents with this difference?</p>