This is a ridiculous feature request and you can do it by dropping to the Mongo driver but it would be downright sick if mongoid supported translating chained atomic commands to form a bigger atomic command.

For example:

@post= Post.find(params[:post_id])
@post.add_to_set(:likers, @current_user.id)
@post.inc(:likes, 1)

Here, all of the commands are individually atomic but not atomic together.

My idea is that this could become:

@post= Post.find(params[:post_id]).add_to_set(:likers, @current_user.id).inc(:likes, 1)

It would be executed in one atomic update.

Tears would be shed if this was possible in ORM language.