node/mongodb: howto move field up one level

I have a collection of a few million documents, with the format:

{
   _id: ...
   main: {
     case: ...
     csc: ...
   }
}

and need it to transform into:

{
  _id: ...
  case: ...
  csc: ...
}

I found an answer in stackoverflow, but there they use the mongo shell (synchronous). I would like to do the same in node using the mongodb driver.

I've tried cursor.each to loop over the documents, but the insert command needs a callback, so I'm stuck with that solution:

coll.find().each(function(err, item) {
  newDoc.insert(item.main, function(err) {
    ???
  })
});

Also I've tried and update:

update({}, {$set: {case: value.case, csc: value.csc}, function(err) {
   ...
});

which doesn't work (as you can not use value.case and value.csc in the $set argument?)

I'm have a solution using find and toArray, looping over documents in nodejs moving the field up one level, and insert the new documents in a new collection:

coll.find().toArray(function(err, documents) {
  for (var doc in documents) {
    newDocs.push(documents[doc].main);
  }
  newColl.insert(newDocs, function(err) {
    ...
  });
});

I'm interested in a maybe more efficient/elegant solution.