rails 4 有很多通过关联查询答案

【问题标题】：rails 4 has many through association queryrails 4 有很多通过关联查询
【发布时间】：2015-07-23 00:20:20
【问题描述】：

我有这些模型，但是当我执行 Message.last.people 或 Message.last.recipient_lists 时出现错误。我将如何引用收件人列表或附加到具有活动记录的邮件的人员？我需要做一个RecipientList.where(:message => Message.last) 吗？似乎应该有更好的方法通过 .message 做到这一点？

class Message < ActiveRecord::Base
    has_many   :people, through: :recipient_list
end

class Person < ActiveRecord::Base
  has_many :messages, through: :recipient_list
end

class RecipientList < ActiveRecord::Base
    belongs_to :person
    belongs_to :message
end

我收到此错误

Message.last.recipient_lists
消息加载 (0.7ms) 选择 "messages".* FROM "messages" ORDER BY "messages"."id" DESC LIMIT 1 NoMethodError: 未定义的方法 `recipient_lists' 用于

来自 /home/nick/.rvm/gems/ruby-2.0.0-p598/gems/activemodel-4.0.9/lib/active_model/attribute_methods.rb:439:in

method_missing' from /home/nick/.rvm/gems/ruby-2.0.0-p598/gems/activerecord-4.0.9/lib/active_record/attribute_methods.rb:168:inmethod_missing' 来自 (irb):1 来自 /home/nick/.rvm/gems/ruby-2.0.0-p598/gems/railties-4.0.9/lib/rails/commands/console.rb:90:in start' from /home/nick/.rvm/gems/ruby-2.0.0-p598/gems/railties-4.0.9/lib/rails/commands/console.rb:9:instart' 来自 /home/nick/.rvm/gems/ruby-2.0.0-p598/gems/railties-4.0.9/lib/rails/commands.rb:62:in <top (required)>' from bin/rails:4:inrequire' 来自 bin/rails:4:in `'

Message.last.people 消息加载 (1.0ms) SELECT "messages".* FROM "messages" ORDER BY "messages"."id" DESC LIMIT 1 ActiveRecord::HasManyThroughAssociationNotFoundError: 找不到关联 :recipient_list in model Message

【问题讨论】：

您没有说明错误。是关于nil 的机会吗？
@DavidHoelzer 编辑了查询 Message.last.recipient_lists 和 Message.last.people 的错误消息
你运行迁移了吗？

标签： ruby-on-rails activerecord has-many-through

【解决方案1】：

您需要在 Message 和 Person 中明确包含 has_many :recipient_lists，并在 through: 选项中正确复数 :recipient_lists，如下所示：

class Message < ActiveRecord::Base
  has_many :recipient_lists
  has_many :people, through: :recipient_lists
end

class Person < ActiveRecord::Base
  has_many :recipient_lists
  has_many :messages, through: :recipient_lists
end

class RecipientList < ActiveRecord::Base
  belongs_to :person
  belongs_to :message
end

【讨论】：