Rails - May 2008 - Complex fields_for mapping to multi-level model association

I''ve come across what might be a limitation of rails, or I''ve
missed
something completely

Here''s the situation:

I have 3 models as follows

class Loan < ActiveRecord::Base
    has_many    :guarantors, :class_name => "Guarantor", :dependent
=> :destroy

    def new_guarantor_attributes=(guarantor_attributes)
      guarantors.build(guarantor_attributes)
    end

    def existing_guarantor_attributes=(guarantor_attributes)
      ...
    end
end

class Guarantor < ActiveRecord::Base
    belongs_to  :loan, :class_name => "Loan", :foreign_key =>
"loan_id"
    has_one     :personal_detail,       :dependent => :destroy

    def personal_detail_attributes=(attributes)
      build_personal_detail(attributes)
    end
end


class PersonalDetail < ActiveRecord::Base
  belongs_to :guarantor, :class_name => "Guarantor", :foreign_key
=>
"guarantor_id"
  # firstname:string
  # middlename:string
  # surname:string
end

All the associations work fine, but populating them does not, in my
form I have a repeating element generated using javascript, similar to
the tasks example found in http://railscasts.com/episodes/75

The partial I am repeating looks something like this
<% fields_for("loan[new_guarantor_attributes][]
[personal_detail_attributes]", object, &block) -%>
  <div class="guarantor">
    <%= f.text_field :firstname %>
    <%= f.text_field :middlename %>
    <%= f.text_field :surname %>

    <%= link_to_function "Remove", ... %>
  </div>
<% end -%>

It works fine if I''m only saving attributes of the guarantor, but its
fails because no matter which configuration I put the first parameter
of fields_for, it either passes an incomplete array, a jumbled array
or in the example above, it passes and empty array which looks like
this:
[{"personal_detail_attributes"=>[]},
{"personal_detail_attributes"=>[]},
{"personal_detail_attributes"=>[]}]
As you can see it does not include any of the text_fields

I would have thought that would pass an array of guarantors each with
a single personal_detail, something like this:
[{"new_guarantor_attributes" =>
[{"personal_detail_attributes"=>[{"firstname"=>"Bill",
"middlename"=>"",
"surname"=>"Bloggs"}]}]},
{"new_guarantor_attributes" =>
[{"personal_detail_attributes"=>[{"firstname"=>"John",
"middlename"=>"",
"surname"=>"Smith"}]}]}]

Here''s the results if I use this config
"loan[new_guarantor_attributes]
[personal_detail_attributes][]"
Inputs:
1: firstname: Bill, :lastname: Bloggs
2: firstname: John, :lastname: Smith
{"personal_detail_attributes"=>[{"firstname"=>"Bill"},
{"middlename"=>"",
"firstname"=>"John"},
{"middlename"=>"",
"surname"=>"Bloggs"},
{"surname"=>"Smith"}]}

As you can see the results are jumpled.

Anyway, to the point, here''s how I need it to work:

Iterate through each guarantor in guarantor_attributes and build it,
and at the same time build each personal_detail (one per guarantor)

Any ideas?

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