Rails 3.2: multiple nested data from one model - ruby-on-rails

Question

How to create form and action for multiple nested attributes if:
LineItem:
has_many :item_options, :dependent => :destroy
has_many :product_options, :through => :item_options
ProductOption:
belongs_to :product
belongs_to :option
has_many :item_options
has_many :line_items, :through => :item_options
ItemOption:
attr_accessible :line_item_id, :product_option_id
belongs_to :line_item, :foreign_key => "line_item_id"
belongs_to :product_option,:foreign_key => "product_option_id"
When I'm creating new LineItem, I need to create new ItemOption(s). This is my form:
<%= form_for(LineItem.new) do |f| %>
<%= f.hidden_field :product_id, value: #product.id %>
<%= f.fields_for :item_options do |io| %>
<% #product.options.uniq.each do |o| %>
<%= o.name %>:
<%= io.collection_select :product_option_id, o.product_options.where(:product_id => #product.id), :id, :value %>
<% end %>
<%= f.submit %>
<% end %>
When I'm clicking on Add To Cart, I've get:
ItemOption(#70296453751440) expected, got Array(#70296430421140)
When Adding accepts_nested_attributes_for :item_options to LineItem, my selects not diplayed :(
With
<%= select_tag "product_option_id", options_from_collection_for_select(o.product_options.where(:product_id => #product.id), :id, :value) %>
#item_options not created:
Parameters: {"utf8"=>"✓", "authenticity_token"=>"/WM5/MqPn1yCxjKWoJQmjfko2pR4RiYV0S2KeTTpA3w=", "line_item"=>{"product_id"=>"1"}, "product_option_id"=>"5", "commit"=>"add"}
And last one, I've create action like this:
#line_item = LineItem.new(params[:line_item])
#line_item.item_options.build
....
Where am I wrong? :( I'm totally confused.
ps. similar question Rails 3.2 has_many through form submission
This is form:

Answer 1

Looks this line:
Parameters: {"utf8"=>"✓", "authenticity_token"=>"/WM5/MqPn1yCxjKWoJQmjfko2pR4RiYV0S2KeTTpA3w=", "line_item"=>{"product_id"=>"1"}, "product_option_id"=>"5", "commit"=>"add"}
The parameter product_option_id is outside line_item hash, and will be inside. Maybe you need write the select like this:
<%= select_tag "line_item[product_option_id]", options_from_collection_for_select(o.product_options.where(:product_id => #product.id), :id, :value) %>
I'm not sure, but maybe is this. Maybe I need more information, like the exact line where is failing.
Extra, the :foreign_key => "line_item_id" and :foreign_key => "product_option_id" are not necesary, because, the belongs_to model name is the same and will use these foreign_key. From api.
Specify the foreign key used for the association. By default this is
guessed to be the name of the association with an “_id” suffix. So
a class that defines a **belongs_to :person** association will use
“person_id” as the default :foreign_key. Similarly, belongs_to
:favorite_person, :class_name => "Person" will use a foreign key of
“favorite_person_id”.
Edit
Sorry, the unknown attribute: product_option_id is because the attribute name is product_option_ids, and is an array, not a unique value. For a has_many relationship, the column name is collection_singular_ids, and the select should be:
<%= select_tag "line_item[product_option_ids][]", options_from_collection_for_select(o.product_options.where(:product_id => #product.id), :id, :value) %>
This should work, I think :)...