ruby-on-rails - 为什么 ActiveRecord 有时只是自动提供相互关联？

Question

在使用 activerecord 时，我注意到在构建关联时，只有在互惠模型已经保存的情况下，才会自动提供互惠关联。如果尚未保存，则必须手动定义此相互关联。我想知道这是故意的还是错误，因为它工作或失败的唯一区别是互惠模型是否已保存。

我希望通过以下要点中描述的三个场景来阐明这一点。

http://gist.github.com/527046

我希望场景 1 在没有场景 3 中显示的额外分配步骤的情况下工作。事实并非如此。

Answer 1

原因很简单——当你通过父对象尚未保存的关联创建对象时，其关联的外键不能有值，因为关联的记录尚未保存。举例说明:

# Registration announcement is not saved at this point, so it has no ID.
@a1 = @e1.build_registration_announcement(:announcement => "Please bring a favorite baked dish.")
# Registration is built without a registration_announcement_id, as @a1 has no ID to pass on.
@a1.registrations.build(:contact_email => "jdoe@yahoo.com", :adults => 1)
#registration doesn't know about its unsaved registration_announcement parent

# Registration announcement is created and saved. ID exists.
@a2 = @e2.create_registration_announcement(:announcement => "Fall house cleaning!")
# Since the Registration announcement has an ID, registration_announcement_id is set on the built associated registration
@a2.registrations.build(:contact_email => "tara.smith@yahoo.com", :adults => 5)
#registration knows about its saved registration_announcement parent

这有点出乎意料，因为您可能逻辑上希望 Rails 保持对象关联，然后在保存时分配关联外键，但它似乎不是那样工作的。