Add/Remove OneToMany collection items w/o persisting?

Rails
1

Ok. Case. I have one duck, which has many ducklings… Some of these
ducklings ceases to exist (dies) or duck gives birth to new ducklings…

However… A duckling is not an empty entity. There are some constraints
in the db which we must conform to. So. Whenever a duck dies or new ones
gets born, details needs to be provided before we actually want to
persist it to the database. So i can’t do:

@duck = Duck.find(:first)
@duck.ducklings << Duck.new

…since i’ve observed that the new duck is persisted right away. This
violates some constraints and i do not want to persist the duck yet.
Furthermore. A duck usually gives birth to more than one duckling at a
time, so we want to manipulate a lot of ducklings from the same form at
the same time. If i remove a duckling, if i add a duckling or if i alter
a duckling’s information, i first want this to be persisted when i hit
update.

Now this has caused me some issues…

Loading the duck and listing form rows for duck.ducklings gives me text
fields with: ducklings[1][name], ducklings[1][birth_date]… That is.
Form submitted back is translated into a hash with key matching the db
id…

However. Now i click add and a new row is created via RJS from a new
instance of Duckling… However. Since this is not persisted, i’ll now
have a row-field with id: ducklings[][name]… Hitting update will now
cause an application error:

Conflicting types for parameter containers. Expected an instance of
Array, but found an instance of Hash. This can be caused by passing
Array and Hash based paramters qs[]=value&qs[key]=value.

Well. I do understand the error message, however. Expected rails to have
some magic in this sense giving me a cleaner view, such that non-keyed
entries were treated as new entries thus would yield a save instead of
update.

Do i really have to distinguish new from existing entries by alternative
field names? Aren’t there any prettier ways to do this?

Haven’t been able to find anyone discussing this particular issue… What
is the rails way? :slight_smile: I’d really like that new and old persisted entries
came back in the same params collection in some way.

2

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.