UPDATE

They way I see it is that you don t have a many to many relationship to the products. You have it to the product of that particulary variety (if any). So in order to keep track of it, I would use an extra model. That way you would be able to keep the relationship between products and varieties as well.

模型:

class User < ActiveRecord::Base
  has_many :user_products, :dependent => :destroy
  has_many :products, :through => :user_products
  has_many :varieties, :through => :user_products
end

class UserProduct < ActiveRecord::Base
  belongs_to :user
  belongs_to :product
  belongs_to :variety
end

class Product < ActiveRecord::Base
  has_many :varieties
end

class Variety < ActiveRecord::Base
  belongs_to :product
end

控制器:

class UserProductsController < ApplicationController
    before_filter do 
        @user = User.find(params[ user_id ])
    end


    def create
      product = Product.find(params[ product_id ])
      variety = Variety.find(params[ variety_id ]) if params[ variety_id ]

      @user_product = UserProduct.new
      @user_product.user = user
      @user_product.product = product
      @user_product.variety = variety
      @user_product.save
    end

    def destroy
        # Either do something like this:
        conditions = {:user_id => @user.id}
        conditions[:product_id] = params[:product_id] if params[:product_id]
        conditions[:variety_id] = params[:variety_id] if params[:variety_id]

        UserProduct.destroy_all conditions
    end
end

The views: If you are not interested in grouping the varieties into the different products and just put them as a list it is enough with this:

# users/show.html.erb
<%= render @user.user_products %>

# user_products/_user_product.html.erb
<%= h user_product.product.name %> 
<%= h user_product.variety.name if user_product.variety %>

Otherwise something a bit more complicated have to be added. Some of it might be possible to put in the model and controller by using association proxies, but I can t help you there

# users/show
<% for product in @user.products do %>
    <%= product.name %>
    <%= render :partial =>  variety , 
        :collection => @user.varieties.find_by_product_id(product.id) %>
<% end %>

# users/_variety
<%= variety.name %> 

当然,把部分内容放在一边是没有必要的(例如,可能是一种微不足道的),但它有助于将不同部分分开,特别是如果你想增加更多的东西。

回答你的问题:

1. Since UserProduct is a join model, you don t really need to keep track of it s individual id. You have the User, the Product and the Variety (in those cases it exists). That s all you need, that combination is unique and thus, you can find the record for deletion. Also, a join model should never be edited (provided that you don t have more attributes in it than these fields), It is either create or delete, since all it do is keep the associations.
2. Since it is basically a viewing thing it is best to place it in the views. You could of course use the controller or the model to collect all the products and the varieties beforehand by doing something like this:

将所有逻辑推向控制员,模型可能不是一个好主意,因为它们不应知道(或谨慎)你想要显示数据。 因此,这只是一个充分准备的问题。

@products_for_user = []
@user.products.each do |product|
 collected_product = {:product => product, 
                      :varieties => @user.varieties.find_by_product_id(product.id)}
  @products_for_user << collected_product
end