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我有關係模型的樣子：更新關係
Foo的has_many bars
而且我有兩個記錄old_cart和new_cart
2.1.2 :014 > old_cart
=> #<Foo id: 1, user_id: nil, created_at: "2015-07-01 05:54:53", updated_at: "2015-07-01 05:54:53">
2.1.2 :015 > new_cart
=> #<Foo id: 2, user_id: 1, created_at: "2015-07-01 05:58:40", updated_at: "2015-07-01 05:58:40">
old_cart有酒吧和new_cart太
2.1.2 :016 > old_cart.bars
Bar Load (0.8ms) SELECT "bars".* FROM "bars" WHERE "bars"."foo_id" = $1 [["foo_id", 1]]
=> #<ActiveRecord::Associations::CollectionProxy [#<Bar id: 3, avatar: nil, created_at: "2015-07-01 05:57:47", updated_at: "2015-07-01 05:57:47", foo_id: 1>]>
2.1.2 :017 > new_cart.bars
=> #<ActiveRecord::Associations::CollectionProxy [#<Bar id: 4, avatar: nil, created_at: "2015-07-01 05:59:07", updated_at: "2015-07-01 05:59:07", foo_id: 2>]>
我想更新old_cart的new_cart.barsfoo_id，併成功：
2.1.2 :018 > new_cart.bars.each do |bar|
2.1.2 :019 > bar.update_attributes(foo_id: old_cart.id)
2.1.2 :020?> end
(0.5ms) BEGIN
SQL (0.8ms) UPDATE "bars" SET "created_at" = $1, "updated_at" = $2, "foo_id" = $3 WHERE "bars"."id" = $4 [["created_at", "2015-07-01 05:59:07.420889"], ["updated_at", "2015-07-01 05:59:07.420889"], ["foo_id", 1], ["id", 4]]
(13.4ms) COMMIT
=> [#<Bar id: 4, avatar: nil, created_at: "2015-07-01 05:59:07", updated_at: "2015-07-01 05:59:07", foo_id: 1>]
foo_id已經改變，但仍然關係不會改變？
2.1.2 :021 > old_cart.bars
=> #<ActiveRecord::Associations::CollectionProxy [#<Bar id: 3, avatar: nil, created_at: "2015-07-01 05:57:47", updated_at: "2015-07-01 05:57:47", foo_id: 1>]>
2.1.2 :022 > new_cart.bars
=> #<ActiveRecord::Associations::CollectionProxy [#<Bar id: 4, avatar: nil, created_at: "2015-07-01 05:59:07", updated_at: "2015-07-01 05:59:07", foo_id: 1>]>