assign a model's attribute through association

Question

I'm new to rails and working on a rails app and I'm stuck pondering this issue.

I have three models

class Product < ActiveRecord::Base

    attr_accessible :name, :issn, :category, :user_products_attributes

    validates_presence_of :name, :issn, :category
    validates_numericality_of :issn, :message => "has to be a number"

    has_many :user_products
    has_many :users, :through => :user_products

    accepts_nested_attributes_for :user_products

end




class UserProduct < ActiveRecord::Base

  attr_accessible :price, :category

  validates_presence_of :price, :category
  validates_numericality_of :price, :message => "has to be a number"

  belongs_to :user
  belongs_to :product

end

class user < ActiveRecord::Base

  # devise authentication here

  # Setup accessible (or protected) attributes for your model
  attr_accessible :email, :password, :password_confirmation, :remember_me

  has_many :user_products, :dependent => :destroy
  has_many :products, :through => :user_products

end

Product controller

def new

@product = product.new

@product.user_products.build 

respond_to do |format|
  format.html # new.html.erb
  format.json { render json: @product }
end

end

So the question is this: I want the user to enter the info for the product in the form but it also involves putting in the price of the product which exists in a different model/table (user_product) that is associated with product. How can I do this? You can see that my form_for uses @product.

Any help will be appreciated.

    <div class="span8">

       <div id="listBoxWrapper">          
        <fieldset>

         <%= form_for(@product, :html => { :class => "form-inline" }, :style => "margin-bottom: 60px" ) do |f| %>


            <div class="control-group">
              <label class="control-label" for="Category">Category</label>
              <div class="controls">

                <%= f.text_field :category, :class => 'input-xlarge', :id => "Category" %>       

              </div>
            </div>

           <div class="control-group" style="display: inline-table;">
           <label class="control-label" for="First Name">Price($/Month)</label>
             <div class="controls">
              <%= product.fields_for :user_products do |p| %>
                <%= p.text_field :price, :class => 'input-xlarge input-name' %>    
                <% end %>
              </div>
            </div>

Answer 1

It should be kind of following

<%= form_for(@product) do |product| %>
  <%= product.text_field :name, :class => 'input-xlarge input-name' %>
  <%= product.fields_for :user_products do |user_product| %>
      <%= user_product.text_field :price, :class => 'input-xlarge input-name' %>        

and you need to build your user_products for the @product in your controller. Like

@product.user_products.build

and your Product model should have the following

accepts_nested_attributes_for :user_products

This will make it realized that user_products values might come while saving a product entity.

Edit:

Just explaining the skeleton of nested form

form_for(@object) do |object_form_builder|
       # call any field generator helper function by object_form_builder like
       object_form_builder.text_field
       object_form_builder.check_box
       # so on...

       #Now for nested forms get the nested objects from the builder like
       object_form_builder.fields_for :nested_objects do |nested_object_builder|
             #generate the fields for this with nested_object_builder. Like
             nested_object_builder.text_field
             # so on...
       end
 end

yes its always convenient to use short names for object builders like you usually use f for form_builder.