简单形式的关联会产生"undefined method `klass' for nil:NilClass"错误

Question

在我的Rails 3应用程序中，我有以下简单的关系结构：

class Rollout < ActiveRecord::Base
    has_many :items, :through => :rollout_items
end
class RolloutItem < ActiveRecord::Base
    belongs_to :rollout
    belongs_to :item
end
class Item < ActiveRecord::Base
    has_many :rollouts, :through => :rollout_items
end

控制器：

def new
    @rollout = Rollout.new
end

我用以下表格得到了上述错误：

<%= simple_form_for @rollout do |f| %>
    <%= f.association :items %>
<% end %>

Answer 1

Rollout和RolloutItem:之间缺少关系

class Rollout < ActiveRecord::Base
    has_many :rollout_items # This.
    has_many :items, :through => :rollout_items
end

Item也是如此。