Rails Model 中的 :through 和 :include 有啥区别?
在 A Guide to Active Record Associations 中描述了 :include 和 :through. 功能看起来是相同的，下面就是两段分别使用 :include 和 :through 的 model 代码，想知道下这两段在功能上有差别吗？
:include
class Supplier < ActiveRecord::Base
has_one :account, :include => :representative
end
class Account < ActiveRecord::Base
belongs_to :supplier
belongs_to :representative
end
class Representative < ActiveRecord::Base
has_many :accounts
end
:through
class Supplier < ActiveRecord::Base
has_one :account
has_one :representative, :through => :account
end
class Account < ActiveRecord::Base
belongs_to :supplier
belongs_to :representative
end
class Representative < ActiveRecord::Base
has_many :accounts
end
跟 find 的 include 参数类似
如果经常要用
account.representative
的话就可以加上 include，这样在查找 account 的时候就会把 representative 查出来并缓存
这样就可以减少数据查询的次数
我是这么理解的
记忆当中是有 inner join 和 left join 的区别 但是试了下，没发现有 inner join 和 left join 的区别 谁来解释下
through 是多对多时的 joint model 比如你上面的例子，Supplier > Account > Representative through Account, 这样 supplier 可以直接找到 rep.
对于 :through
s1 = Supplier.create
r1 = Representative.create
Account.create( :supplier_id => s1.id, :representative_id => r1.id)
Supplier.first.representative
> Supplier Load (0.3ms) SELECT "suppliers".* FROM "suppliers" LIMIT 1
> Representative Load (0.2ms) SELECT "representatives".* FROM "representatives" INNER JOIN "accounts" ON "representatives"."id" = "accounts"."representative_id" WHERE "accounts"."supplier_id" = 1 LIMIT 1
> => #<Representative id: 1, name: "rep1", created_at: "2012-04-21 00:12:53", updated_at: "2012-04-21 00:12:53">
对于 :include
似乎这时候 Supplier 根本就不存在 representative 方法，必须绕圈来读取
Supplier.first.account.representative
> Supplier Load (0.2ms) SELECT "suppliers".* FROM "suppliers" LIMIT 1
> Account Load (0.2ms) SELECT "accounts".* FROM "accounts" WHERE "accounts"."supplier_id" = 1 LIMIT 1
> Representative Load (0.1ms) SELECT "representatives".* FROM "representatives" WHERE "representatives"."id" IN (1)
> => #<Representative id: 1, name: "rep1", created_at: "2012-04-21 00:12:53", updated_at: "2012-04-21 00:12:53">
model 就是按照主贴内容写的，最后的 SQL 没有各位所说的 join 啊什么的..咋回事呢
:includes 屬於查詢效能優化的問題 執行：
A.limit(10).map{ |a| a.b}
這時候會產生 11 (1 + 10) 次查詢，第一次是 10 筆 A 的查詢，後面逐一對 a 的 b 又要個別查詢，這會產生 10 次 b 的查詢
若加入 :includes
A.includes(:b).limit(10).map{ |a| a.b}
則只會有兩次 sql 查詢，一次是針對 A 本身 (取出 10 筆)，另一次會直接以 10 筆 a 的 id，用 in(a1.id, a2.id....) 作為條件直接取得 10 筆 b，這樣只有兩次查詢。由 11 次減為 2 次。