title: Pymongo API TypeError: Unhashable dict
body: <p>I'm writing an API for my software so that it is easier to access mongodb.</p>
  
  <p>I have this line:</p>
  
  <pre><code>def update(self, recid):        
      self.collection.find_and_modify(query={"recid":recid}, update={{ "$set": {"creation_date":str( datetime.now() ) }}} )
  </code></pre>
  
  <p>Which throws <code>TypeError: Unhashable type: 'dict'</code>.</p>
  
  <p>This function is simply meant to find the document who's recid matches the argument and update its creation_date field.</p>
  
  <p>Why is this error occuring?</p>