Possible Duplicate:
Update MongoDB field using value of another field
For database migrations I always found it helpful to issue something like this in SQL:
UPDATE users SET full_name = users.first_name || " " || users.last_name
(PL/SQL)
or
UPDATE users SET full_name = CONCAT( users.first_name, users.last_name )
(MySQL)
How do I do this in MongoDB without fetching all documents first and update them by iteration?