Mass-assign protected attributes (Nested-form) Ruby/Rails - ruby-on-rails

Question

I'm trying to create a nested form but i got this error when trying to assign the parameters. I read a bunch of similar posts but cant figure out the problem. What could be wrong?
Can't mass-assign protected attributes: detalle_poliza
My models:
poliza_contable.rb
class PolizaContable < ActiveRecord::Base
has_many :detalle_polizas
accepts_nested_attributes_for :detalle_polizas
attr_accessible :concepto_poliza, :estatus, :fecha_aplicacion, :fecha_poliza, :no_poliza, :tipo, :tota_de_cargos, :total_de_abonos
end
detalle_poliza.rb
class DetallePoliza < ActiveRecord::Base
belongs_to :cuenta_contable
belongs_to :poliza_contable
attr_accessible :abono, :cargo,:cuenta_contable_id, :poliza_contable_id, :user_id, :id, :updated_at, :created_at
end
My form:
<%= form_for #poliza_contable, :html => { :class => 'form-horizontal' } do |f| %>
## OTHER FIELDS
<%= f.fields_for :detalle_poliza_attributes do |builder| %>
<% render :partial => "detalle_polizas/form", :locals => { :f => builder } %>
<% end %>
<% end %>
Rendered form:
<div class="control-group">
<%= f.label :cargo, :class => 'control-label' %>
<div class="controls">
<%= f.text_field :cargo, :class => 'text_field' %>
</div>
</div>
<div class="control-group">
<%= f.label :abono, :class => 'control-label' %>
<div class="controls">
<%= f.text_field :abono, :class => 'text_field' %>
</div>
</div>
<div class="control-group">
<%= f.label :poliza_contable_id, :class => 'control-label' %>
<div class="controls">
<%= f.number_field :poliza_contable_id, :class => 'number_field' %>
</div>
</div>
<div class="control-group">
<%= f.label :cuenta_contable_id, :class => 'control-label' %>
<div class="controls">
<%= f.number_field :cuenta_contable_id, :class => 'number_field' %>
</div>
</div>
<div class="control-group">
<%= f.label :user_id, :class => 'control-label' %>
<div class="controls">
<%= f.number_field :user_id, :class => 'number_field' %>
</div>
</div>
Request Parameters:
{"utf8"=>"✓",
"authenticity_token"=>"mI23Nnj4oPX+IW3mCvvIV7Auij+pjX/a7bl/HsudEW8=",
"poliza_contable"=>{"tipo"=>"Diario",
"concepto_poliza"=>"",
"fecha_poliza"=>"2012-06-25",
"detalle_poliza"=>{"cargo"=>"34",
"abono"=>"34",
"poliza_contable_id"=>"34",
"cuenta_contable_id"=>"34",
"user_id"=>"1"}}
"commit"=>"Create Poliza contable"}`
I'll appreciate any comment to fix it.

Answer 1

With according to railscast 196 Nested Model Form Part 1 you need to allow DetallePoliza attributes to be saved with parent. To achieve this goal just add attr_accessible :detalle_polizas_attributes to PolizaContable model:
poliza_contable.rb
class PolizaContable < ActiveRecord::Base
has_many :detalle_polizas
accepts_nested_attributes_for :detalle_polizas
attr_accessible :concepto_poliza, :estatus, :fecha_aplicacion, :fecha_poliza, :no_poliza, :tipo, :tota_de_cargos, :total_de_abonos, :detalle_polizas_attributes
end

Answer 2

Just making some research, finally found the nestes_form gem
Its a very straight forward implementation!
Hope it helps someone.