title: How to know if some data have no been extracted
body: <p>I am extracting data from a collection in mongodb. my question is: can I somehow know a document that have no been extracted, or eve, this can happen? because I am writing every document into a file and i want to be sure that everything is there, another doubt is... Is the process of extracing data fail in a certain point can I continue the process in the point of failure? I have been checking "natural order" to manage to know where i should be continuing the process but it seems it is not an reliable order.</p>