I am sorry for asking a beginner s question. I can t get a "collection_select" to work in a specific case.

试图根据以下模式撰写一条简单的铁路3.1:

class Site < ActiveRecord::Base
     has_many :supply_sites, :dependent => :destroy
     has_many :demand_sites, :dependent => :destroy
     accepts_nested_attributes_for :supply_sites
     accepts_nested_attributes_for :demand_sites
end


class DemandSite < ActiveRecord::Base
   belongs_to :site, :class_name => "Site"
   has_many :translinks , :dependent => :destroy
end


class SupplySite < ActiveRecord::Base
   belongs_to :site, :class_name => "Site"
   has_many :translinks , :dependent => :destroy
end


class Translink < ActiveRecord::Base
  belongs_to :supply_site, :class_name =>  "SupplySite"
  belongs_to :demand_site, :class_name =>  "DemandSite"
end

Migrations are as follows:

class CreateSites < ActiveRecord::Migration
  def self.up
    create_table :sites do |t|
      t.string :name
      t.string :codename, :limit => 3

      t.timestamps
    end
  end

  def self.down
    drop_table :sites
  end
end



class CreateSupplySites < ActiveRecord::Migration
  def self.up
    create_table :supply_sites do |t|
      t.integer :site_id
      t.float :supply_quantity

      t.timestamps
    end
  end

  def self.down
    drop_table :supply_sites
  end
end


class CreateDemandSites < ActiveRecord::Migration
  def self.up
    create_table :demand_sites do |t|
      t.integer :site_id
      t.float :demand_quantity

      t.timestamps
    end
  end

  def self.down
    drop_table :demand_sites
  end
end


class CreateTranslinks < ActiveRecord::Migration
  def self.up
    create_table :translinks do |t|
      t.integer :supply_site_id
      t.integer :demand_site_id
      t.float :unit_cost
      t.float :quantity

      t.timestamps
    end
  end

  def self.down
    drop_table :translinks
  end
end

I want to be able to add a new "translink" (Transportation Link) between a "Supply Site" and a "Demand Site by selecting from a Drop-Down menu based on the Codenames of the respective Supply Site or Demand Site, where the codenames are specfified for the "Sites".

When adding a new "Supply Site" (or a "Demand Site"), the following works well (from "_form.html.erb" for either Supply Sites or Demand Sites.

<div class="field">
    <%= f.label :site_id %><br />
    <%= f.collection_select :site_id, Site.find(:all), :id, :codename %>
  </div>

Now I want something analogous for for adding a new "Translink" connecting a Supply Site and a Demand Site. I don t want to add the respective supply_site_id or demand_site_id manually, but a list of all supply sites to select one by the codename defined in the underlying site, and the same for the demand sites. I can do the following to to have a drop-down menu to select, say, one out of the existing supply sites:

<div class="field">
    <%= f.label :supply_site_id %><br />
    <%= f.collection_select :supply_site_id,  SupplySite.find(:all), :id, :id  %>
</div>

然而,我不但没有在下降的菜单中显示供应点的宽度,而是会看到并选择基本“现场”的代码名称。

<div class="field">
    <%= f.label :supply_site_id %><br />
    <%= f.collection_select :supply_site_id,  SupplySite.find(:all), :id ,????? %>
  </div>

How can I do this?

任何帮助都将受到高度赞赏。

Stefan