Rails - Nov 2009 - Has Many Relationship: collection_ids= Method Useless?

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.

On Sat, Nov 14, 2009 at 6:37 AM, MaggotChild
<hsomob1999-/E1597aS9LQAvxtiuMwx3w@public.gmane.org>
wrote:
>
> 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?
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.
-- 
Leonardo Mateo.
There''s no place like ~

On Nov 14, 2:42 am, Leonardo Mateo
<leonardoma...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> On Sat, Nov 14, 2009 at 6:37 AM, MaggotChild
<hsomob1...-/E1597aS9LQAvxtiuMwx3w@public.gmane.org> wrote:
> > 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?
>
> 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]
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.