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我有以下课程：

class Car < ActiveRecord::Base

  has_many  :adverts, :dependent => :destroy
  has_many  :active_adverts, :class_name => 'Advert', :select => "adverts.id, adverts.availability_state, adverts.source_name, adverts.last_observed_at", :conditions => "availability_state = 'active'"

end

当我这样做时：cars = Car.where(:brand => 'BMW').includes(:adverts)它工作正常，并返回我请求的汽车数组，并且广告渴望正确加载。

如果我这样做：cars = Car.where(:brand => 'BMW').includes(:active_adverts)它似乎执行了预期的查询，但它返回了一个ActiveRecord::Relation对象，如果我问cars.first我得到：

NoMethodError: undefined method `each' for nil:NilClass
from /Users/nk/.rvm/gems/ruby-1.9.3-p194@au/gems/activerecord-3.2.13/lib/active_record/associations/preloader/association.rb:88:in `block in associated_records_by_owner'

我究竟做错了什么？

PS。我听从了她的建议：http: //api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html

它在哪里声明：

如果您确实希望只加载关联的某些成员，通常更自然地包含一个定义了条件的关联：

class Post < ActiveRecord::Base
  has_many :approved_comments, :class_name => 'Comment', :conditions => ['approved = ?', true]
end

Post.includes(:approved_comments) 这将加载帖子并急切加载approved_comments 关联，该关联仅包含已批准的评论。

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1 回答 1

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找到了，忘记在select中选择外键car_id。

于 2013-05-24T09:50:06.697 回答