title: MongoDB: Chain find()
body: <p>In MongoDB how do you chain <code>find()</code> methods? Suppose I want to do something like this in Python/pymongo</p>
  
  <pre><code>query = prices.find({'symbol': "AAPL"})
  if start is not None:
      query = query.find({'date': {'$gte': start}})
  if end is not None:
      query = query.find({'date': {'$lte': end}})
  </code></pre>