ruby on rails - HABTM with extra attributes on nested form -

i have products have , belong many product_links

class product < activerecord::base  has_many :map_product_prices has_many :product_links, :through => :map_product_prices accepts_nested_attributes_for :product_links 

and...

class productlink < activerecord::base  has_many :map_product_prices has_many :products, :through => :map_product_prices accepts_nested_attributes_for :products 

i using ryan bates 'nested_form' gem this. here problem cannot wrap brain around. map table has price attribute attached also.

class mapproductprice < activerecord::base  attr_accessible :product_link_id, :product_id, :price  belongs_to :product belongs_to :product_link 

i had multi-select box working select 1 or many products while on product link form. when switched on ryans gem use nested forms have single select box choose product, text field input price. can add / delete them necessary.

here i've got in view:

    <%= f.fields_for :products |product| %>         <%= product.select :product_id, options_for_select(select_options) %>         <%= product.text_field :price %>         <%= product.link_to_remove "remove product" %>     <% end %>     <p><%= f.link_to_add "add product", :products %></p> 

anyone have ideas on how best accomplish this?

what i'm getting form there no product_id attribute product. makes sense because product_id on mapping table, not product. how can reference mapping table here in nested form instead of product? don't want able add new products, want able add new mappings existing products addition of price next each mapping.

thanks

edit: have edited code reflect has many through tried. when @ view, tells me there no product_id or price attribute on product builder object. how translate design onto form , make work? thanks

you need switch habtm has_many :through relationship. habtm table can have 2 foreign key fields, has_many :through join model has primary key, , can can add many fields want join model.

rails guide: here

a example of when want use has_many : throughrelationship join model adding exercises workout:

class workout < activerecord::base    has_many :workout_exercises,    has_many :exercises, :through => :workout_exercises  class workoutexercise < activerecord::base    belongs_to :exercise   belongs_to :workout  class exercise < activerecord::base    has_many :workout_exercises   has_many :workouts, :through => :workout_exercises 

so exercise within workout can have weight, or whatever else want associated adding weight field join table.