title: Cannot update document in PyMongo
body: <p>I have code in Python/PyMongo like ( code should simulate inner join and all documents from second collection to put (nest inside) in list and add to first appropriate document in Mongo). Idea is to load two collections and pass two fields ( name of fields for connection ) and all documents from second collection with same attribute like in first put in list and add to appropriate document  in first collection .</p>
  
  <p><strong>(For example in first collection I have documents with fileds "country","population" and in second I have "country" and "car factories" and I want to put ( denormalize ) first collection list of all factories for appropriate country)</strong> </p>
  
  <pre><code>    for f in first_collection_records:
          temp=[]
          id=f['_id']
          second_collection_records.rewind()
          for s in second_collection_records:
              if f[field_one]==s[field_two]:
                  temp.append(s)
          f[self.__second_collection_name__]=temp
          first_collection_records.update({"_id":id}, f, safe=True)
  </code></pre>
  
  <p>but I got error 'Cursor' object has no attribute 'update'. What I done wrong ?</p>