（自動生成？）_ids提供ActiveRecord :: UnknownPrimaryKey錯誤

Question

假設我有模型

class Widget < ActiveRecord::Base
    has_many :widget_spots 

和

class WidgetSpot < ActiveRecord::Base
  belongs_to :widget

另外，在我的MySQL表“ widget_spots”中，我有一列“ widget_id”

+--------------------+------------+------+-----+---------+-------+
| Field              | Type       | Null | Key | Default | Extra |
+--------------------+------------+------+-----+---------+-------+
| id                 | int(11)    | NO   | PRI | NULL    |       |
| widget_id          | int(11)    | NO   | PRI | NULL    |       |
                ................
+--------------------+------------+------+-----+---------+-------+

現在，如果在我的Rails控制台中，嘗試訪問widget_spot_ids會出現錯誤。

> widget = widgets.find(:first)
> widget.widget_spot_ids
ActiveRecord::UnknownPrimaryKey: ActiveRecord::UnknownPrimaryKey
    from /Users/xxxxx/.rvm/gems/ruby-1.9.2-p320@rails-3.1.1/gems/activerecord-3.1.1/lib/active_record/reflection.rb:374:in `primary_key'
    from /Users/xxxxx/.rvm/gems/ruby-1.9.2-p320@rails-3.1.1/gems/activerecord-3.1.1/lib/active_record/reflection.rb:228:in `association_primary_key'
    from /Users/xxxxx/.rvm/gems/ruby-1.9.2-p320@rails-3.1.1/gems/activerecord-3.1.1/lib/active_record/associations/collection_association.rb:51:in `ids_reader'
    from /Users/xxxxx/.rvm/gems/ruby-1.9.2-p320@rails-3.1.1/gems/activerecord-3.1.1/lib/active_record/associations/builder/collection_association.rb:62:in `block in define_readers'
    from (irb):12
    from /Users/xxxxx/.rvm/gems/ruby-1.9.2-p320@rails-3.1.1/gems/railties-3.1.1/lib/rails/commands/console.rb:45:in `start'
    from /Users/xxxxx/.rvm/gems/ruby-1.9.2-p320@rails-3.1.1/gems/railties-3.1.1/lib/rails/commands/console.rb:8:in `start'
    from /Users/xxxxx/.rvm/gems/ruby-1.9.2-p320@rails-3.1.1/gems/railties-3.1.1/lib/rails/commands.rb:40:in `<top (required)>'
    from script/rails:6:in `require'
    from script/rails:6:in `<main>'

但是，獲取實例會給出一個沒有錯誤的列表： > widget.widget_spots執行預期的SQL查詢並給出一個列表。

如何確定在哪里以及如何生成ActiveRecord::UnknownPrimaryKey錯誤。 它試圖在哪個數據庫中找到哪個主鍵？

Answer 1

這可能是因為目標widget_spots表的主鍵由多個字段組成。