Taken from https://github.com/meteor/meteor/blob/master/examples/parties/model.js

gistfile1.txt

    if (Meteor.isServer) {
        // update the appropriate rsvp entry with $
        Parties.update(
            {_id: partyId, "rsvps.user": this.userId},
            {$set: {"rsvps.$.rsvp": rsvp}});
        } else {
            // minimongo doesn't yet support $ in modifier. as a temporary
            // workaround, make a modifier that uses an index. this is
            // safe on the client since there's only one thread.
            var modifier = {$set: {}};
            modifier.$set["rsvps." + rsvpIndex + ".rsvp"] = rsvp;
            Parties.update(partyId, modifier);
        }
    }

I would need some help on the "Black Mongo Magic" that is going on here:

1. What is the $ in line 5? An index? But how is it defined? Where can I find more details on that in the mongo docs?
2. How is the workaround on the client side working? A $set on a $set? 😕

1. What is the $ in line 5? An index? But how is it defined? Where can I find more details on that in the mongo docs?

http://docs.mongodb.org/manual/reference/operator/positional/

2. How is the workaround on the client side working? A $set on a $set?

Looks like a workaround for the minimongo library's overly strict validation of find() and update() arguments. Perhaps it complains about using $ as a component in a field path.