Has Many Relationship: collection_ids= Method Useless?

rubyonrails-talk
#1

When modifying a model that has a has_one or has_many relationship
people usually create form that will submit the id of the selected
associated object. Rails takes this into consideration with
validations: validate_presence_of :country will validate if country
or country_id has been set.

In Rails, providing the associated ID works fine with has_one but if
you try to do this with a has_many:

form_for(@country) do |form|
  form.select :state_ids, @states
  form.select :state_ids, @states
  #etc...

The selected state_ids are written to the DB as soon as they're
assigned. What's the reasoning behind this?
How is one supposed to assign multiple collection ids via a form?

accepts_nested_attributes_for doesn't work out too well either because
you're just setting IDs -a field that is explicitly ignored by
accepts_nested except when testing whether or not the given params are
new.

#2

You have to send them as an array, and assign them as an array too.

For example,
View
form.select("state_ids[]", @states)
form.select("state_ids[]", @states)

Controller
@whatever.state_ids = params[:state_ids]

You'll have to figure out the exact syntax, but that's the idea.

Hope it helps.

#3

That's the problem. As soon as you make that assignment the IDs are
written to the db. You'd have to say:
Whatever.tranaction do
  @what = Whatever.new(params[:whatever])
  #other stuff
  if @what.valid? ....
end

Which can result in an unnecessarily large transaction scope. Though
this is somewhat expected since << and co. also write to the DB on
assignment.