mongodb looping collection + save, objects returned several times - mongodb

Question

I'm writing a pretty big migration and had this code (coffeescript):
db.users.find().forEach (user)->
try
#some code changing the user depending on the old state
db.users.save(user)
print "user_ok: #{user._id}"
catch error
print "user_error: #{user._id}, error was: #{error}"
Some errors occured. But they occured on already processed users:
user_ok: user_1234
#many logs
user_error: user_1234 ...
How come the loop takes already processed objects?
I ended up doing:
backup = { users: [] }
db.users.find().forEach (user)->
try
#some code changing the user depending on the old state
backup.users.push user
print "user_ok: #{user._id}"
catch error
print "user_error: #{user._id}, error was #{error}"
#loop backup and save
And it works nice now, but it seems really weird. What's the point behind all that please?

Answer 1

When you modify an object, it might be moved by the database. The database needs to take additional care to remember which objects have been visited already. This feature is called snapshotting, you can ask for a snapshotted query using
db.collection.find().snapshot()
However, even this doesn't make guarantees about objects that were inserted or deleted during the cursor iteration. A few more caveats are explained in the link to the documentation.
Another option is to perform an $orderby on an invariable unique index. Ideally, that index is also monotonic, so if you are using ObjectIds as primary keys then the _id field comes in pretty handy, like
db.collection.find().sort({"_id" :1});