Rails - Sep 2005 - select list using a the method of a method

I''m having, what appears to me to be, a bit of a complicated
select/collection_select helper problem.

Product belongs to SubCategory, which belongs to Category.

The form I''m using to edit a product has a collection_select that
draws from the product''s sub_category method:

<% @sub_categories = SubCategory.find(:all, :conditions => [
"category_id = ?", cat ]) %>
<%= collection_select(''product'',
''sub_category_id'', @sub_categories,
''id'', ''name'') %>

cat is a local variable passed to the partial that is drawn from
@product.sub_category.category.id

This collection_select automatically selects the product''s
sub_category.

I also need a select list that gives all the categories and that
automatically pre-selects the correct category for that product.
Because the product belongs_to SubCategory, and SubCategory belongs to
Category, I can''t work out how to reference the category''s id
as the
method in the collection_select helper. I''ve tried a few things,
including:

<% @categories = Category.find(:all) %>
<%= collection_select(''sub_category'',
''category_id'', @categories,
''id'', ''name'') %>

That, however, doesn''t pre-select the product''s category.

I''m sure I must be able to reference the category from the product
object, even though it''s once removed via the sub_category method.
I''ve tried putting:

<% @categories = Category.find(:all) %>
<%= collection_select(''product'',
''sub_category_category_id'',
@categories, ''id'', ''name'') %>

and similar things but it throws up an unknown method error.

To recap:

I want a drop-down list of categories, with the current product''s
category pre-selected. However, the product belongs_to a sub_category,
which in turn belongs_to a category. The product does not have a
direct belongs_to relationship with the category.

Oh, and I only plan on using the category selection list to update
sub_category selection list with the correct sub_categories through an
observe_field helper.

Any thoughts anyone?

Thanks :)

--
Matthew Revell
www.understated.co.uk

Matthew Revell wrote:
> <% @categories = Category.find(:all) %>
> <%= collection_select(''sub_category'',
''category_id'', @categories,
> ''id'', ''name'') %>
> 
> That, however, doesn''t pre-select the product''s category.
> 
> I''m sure I must be able to reference the category from the product
> object, even though it''s once removed via the sub_category method.
> I''ve tried putting:
> 
> <% @categories = Category.find(:all) %>
> <%= collection_select(''product'',
''sub_category_category_id'',
> @categories, ''id'', ''name'') %>
> 
> and similar things but it throws up an unknown method error.
Perhaps using the compound objects in form helpers patch
(http://dev.rubyonrails.com/ticket/2053) you can write:

<%= collection_select(''product[sub_category]'',
''category_id'', @categories, ''id'',
''name'') %>

But if this doesn''t work, either give product an attr_accessor
''category_id''
method, or build the options manually using options_from_collection_for_select:

<%= select_tag(''product[category_id]'',
options_from_collection_for_select(@categories, ''id'',
''name'', @product.sub_category.category_id)) %>


-- 
We develop, watch us RoR, in numbers too big to ignore.