ActiveRecord具有自定义外键sql表达式(_many)

Question

在ActiveRecord中，has_many关系使用外键列来加载关联。因此：

class Person < ActiveRecord::Base
  has_many :messages
end
class Messages < ActiveRecord::Base
  belongs_to :person
end
Person.limit(10).includes(:messages) 
# select * from persons limit 10;
# select messages.* from messages where person_id in (1, 2, 3...)

我有一个案例（我也看到其他人要求这样做），我不希望Rails自动将外键检查附加到where子句。相反，我可能想要这样的东西：

class Person < ActiveRecord::Base
  has_many :messages, 
           :foreign_key => false, 
           :conditions => proc { ["person_id is null or person_id = ?", self.id] }
end
Person.limit(10).includes(:messages)
# select messages.* from messages where person_id is null or person_id in (1, 2, 3...)

我该怎么做？总之，我不希望ActiveRecord在WHERE子句中自动附加外键，我希望能够指定它用于关联的表达式。

我不想这样做：

class Person < ActiveRecord::Base
  def messages
    Message.where("person_id is null or person_id = #{ self.id }")
  end
end

据我所知，这会打破急切的负载。

我也不想使用finder_sql选项来has_many，因为这会破坏像person.messages.where(:id => 1)或Person.limit(10).includes(:messages => :image) 这样的东西

Answer 1

类似的东西在Rails3.2.1:上对我有效

class Person < ActiveRecord::Base
  has_many :messages, :finder_sql => lambda{ "SELECT * FROM messages WHERE messages.person_id=#{id} OR messages.person_id IS NULL" }
end
# person = Person.includes(:messages).first
# person.messages.loaded?  #=> true