I have a collection with a few 100k records on a replica set.

When I update all records to set :send_weekly_emails => false:

Person.collection.update({}, { '$set' => { :send_weekly_emails => false }}, { :multi => true })

Then I query (in a separate rails console) to see if things have gone through and it takes several minutes before all of the people have the flag set to false.

> Person.where(:send_weekly_emails => true).count # => 56
> Person.where(:send_weekly_emails => true).count # => 38
> Person.where(:send_weekly_emails => true).count # => 37
> Person.where(:send_weekly_emails => true).count # => 2
> Person.where(:send_weekly_emails => true).count # => 2
> Person.where(:send_weekly_emails => true).count # => 2
> Person.where(:send_weekly_emails => true).count # => 2
> Person.where(:send_weekly_emails => true).count # => 0

Isn't the multi update supposed to be atomic? If anything, I would not expect it to finish before the changes were actually persisted to the replica set.

Note: the send_weeky_emails field is not indexed.