Add/Remove OneToMany collection items w/o persisting?

A few thoughts:

The first thing is to understand when Rails will automatically persist
records. In a 1:n relationship where a Duck has_many Ducklings and a
Duckling belongs_to Duck, the foreign key is stored in the Duckling.
If you assign a Duckling to Duck it will persist it right away because
it assumes you are working on the Duck and their is just a single
change to the Duckling that must be saved right away. If you want to
make the association without saving, you assign @duckling.duck_id =
@duck.id because that appears you are primarily editing the Duckling
and will manage your own saving. The first scenario is really just a
convience since you would have to individually save all the ducklings
anyway, and if you forget, Rails has no efficient way of remember
which to save when you do @duck.save.

Regarding your second scenario, there is no magic I'm afraid. I could
definitely see some improvements being made to support this kind of
thing, but ultimately I think you can roll something up that's pretty
clean.

The first thing is the whole concept of objects[][name]&objects[][age]
is failed even in the absence of a conflicting hash because there's no
way to keep the groups coherent. I suppose this could be fixed as
long as you can guarantee the presence of every field (been burned by
checkboxes in this situation in PHP). But really if you have more
than one, you need to index them somehow.

So you can stick your own index in there to make it a hash. But you
will also need to distinguish the new from the updates. So it should
have a different name such as new_ducklings[0], new_ducklings[1].
Then you can create a new_ducklings=(hash) method on the Duck model
that processes these accordingly.

However, if you are just adding one at a time using RJS, you can just
skip the whole index, and have a plain new_duckling hash of fields.

Hopefully that will help a bit. It's definitely not a trivial
problem.