渴望加载条件关联

Question

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)可以正常工作，并向我返回我请求的汽车数组，并且广告渴望正确加载。

我怎么办： cars = Car.where(:brand => 'BMW').includes(:active_adverts)它似乎执行了预期的查询，但是它返回了一个ActiveRecord::Relation对象，并且问我是否要问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。 我遵循她的建议： http : //api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html

说明：

如果您确实希望只加载某个关联的某些成员，则通常更自然的做法是包括一个已定义条件的关联：

 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，并急于加载validated_comments关联，该关联仅包含已批准的注释。

Answer 1

找到它，忘记在选择中选择外键car_id 。