我创建了一个示例应用程序来启发我的问题。
基本上，我与 STI 类有一个多对多的关联：
class Basket < ActiveRecord::Base
has_many :basket_fruits, inverse_of: :basket
has_many :fruits, through: :basket_fruits
has_many :apples, through: :basket_fruits, source: :fruit, class_name: 'Apple'
has_many :oranges, through: :basket_fruits, source: :fruit, class_name: 'Orange'
has_many :bananas, through: :basket_fruits, source: :fruit, class_name: 'Banana'
end
class BasketFruit < ActiveRecord::Base
belongs_to :basket, inverse_of: :basket_fruits
belongs_to :fruit, inverse_of: :basket_fruits
validates :basket, :fruit, presence: true
end
class Fruit < ActiveRecord::Base
has_many :basket_fruits, inverse_of: :fruit
has_many :baskets, through: :basket_fruits
validates :type, presence: true, inclusion: ['Apple', 'Banana', 'Orange']
validates :baskets, presence: true
end
class Apple < Fruit; end
class Banana < Fruit; end
class Orange < Fruit; end
架构：
Basket(id: integer, created_at: datetime, updated_at: date time)
BasketFruit(id: integer, fruit_id: integer, basket_id: integer, created_at: datetime, updated_at: date time)
Fruit(id: integer, type: string, created_at: datetime, updated_at: datetime)
当使用 ActiveAdmin 创建新 Banana 时，它使用的代码相当于：
Basket.create
banana = Basket.first.bananas.build(basket_ids: [Basket.first.id])
出现的问题是这段代码创建了两个关联，即使不保存香蕉也可以观察到：
banana.basket_fruits.size
=> 2
为什么会发生这种情况，以及如何解决？
更新
才发现这个。也许这就是为什么 AR 试图两次添加关系，而不是像我预期的那样“覆盖”：
banana = Basket.first.bananas.build
=> #<Banana id: nil, type: "Banana", created_at: nil, updated_at: nil>
banana.baskets
=> #<ActiveRecord::Associations::CollectionProxy []>
在这种情况下，需要在调用后预先填充关联#build