Save only working on New objects


I’m using a habtm self referential (below). When I work with it in the
console, if I do this:

memberA.friends[0] = memberB

Nothing actually gets saved to the join table. (memberA and memberB are
previously saved objects).

If, however, I do this:

memberA.friends[0] = // the new object here is the

the relationship Does get saved to the join table.

Anyone have a clue as to why this is? And how I can get changes to
previously saved objects to save to the sql join table?