Executes collection's update method based on keyword args.

        Example::

            manager = EntityManager(Product)
            p = Product()
            p.name = 'new name'
            p.description = 'new description'
            p.price = 300.0

            yield manager.update(p)

def update(self, entity):
        """ Executes collection's update method based on keyword args.

        Example::

            manager = EntityManager(Product)
            p = Product()
            p.name = 'new name'
            p.description = 'new description'
            p.price = 300.0

            yield manager.update(p)

         """
        assert isinstance(entity, Entity), "Error: entity must have an instance of Entity"
        return self.__collection.update({'_id': entity._id}, {'$set': entity.as_dict()})