除非“包含”，否则不会加载活动记录关联。从原来的查找

Question

为什么 client_destination 关联返回 nil，除非它是从原始模型查找中“包含”的！？

class Placement < ActiveRecord::Base
  belongs_to :client_destination_type

  belongs_to :client_destination,
    :polymorphic => true,
    :foreign_type => 'client_destination_type_class_name'

  def client_destination_type_class_name
    client_destination_type.class_name
  end
end

不包含（损坏）：

ruby-1.9.2-p0 :021 > Placement.last.client_destination
  Placement Load (0.5ms)  SELECT `placements`.* FROM `placements` WHERE (placements.deleted_at IS NULL) ORDER BY `placements`.`id` DESC LIMIT 1
 => nil 

包含（有效）：

ruby-1.9.2-p0 :022 > Placement.last(:include => :client_destination).client_destination
  Placement Load (0.7ms)  SELECT `placements`.* FROM `placements` WHERE (placements.deleted_at IS NULL) ORDER BY `placements`.`id` DESC LIMIT 1
  ClientDestinationType Load (0.4ms)  SELECT `client_destination_types`.* FROM `client_destination_types` WHERE `client_destination_types`.`id` = 2 LIMIT 1
  ClientSite Load (0.4ms)  SELECT `client_sites`.* FROM `client_sites` WHERE `client_sites`.`id` IN (3000018)
 => #<ClientSite id: 3000018, name: "WTF.com", created_at: "2011-08-09 12:23:51", updated_at: "2011-08-09 12:23:51", user_group_id: 23510, client_ad_tag_code_id: nil> 

Answer 1

我无法从您发布的内容中看出，但我猜想这是您在默认加载时行为不当的动态foreign_type选择器。

我猜测加载 client_destination_type 关联的操作顺序是为了生成其他关联中使用的 client_destination_type_class_name() 方法的值。

您可以通过换出：

belongs_to :client_destination,
  :polymorphic => true,
  :foreign_type => 'ClientSite'

并查看该静态类型的关联延迟加载是否正确来确认或否认这一点。

Answer 2

这修复了它（默认范围）：

class Placement < ActiveRecord::Base
  belongs_to :client_destination_type

  default_scope :joins => :client_destination_type

  belongs_to :client_destination,
    :polymorphic => true,
    :foreign_type => 'client_destination_types.class_name'
end