Rails里面includes是比较常用的 依旧先码出背景
class LineItem < ActiveRecord::Base
belongs_to :order, -> { includes :customer }
end
class Order < ActiveRecord::Base
belongs_to :customer
has_many :line_items
end
class Customer < ActiveRecord::Base
has_many :orders, -> { includes :line_items }
end
关联 includes方法指定使用关联时要按需加载的间接关联
belongs_to
@line_item.order.customer
如经常需要从 line_item上取到需要的 Customer 则这个 belongs_to :order 关联就有必要加上 includes
has_many & has_one
@customer.orders.line_items
如果经常需要从 customer 上取到 line_items 则最后这个 has_many :orders 关联就有必要加上includes
没有加入includes时候的sql
2.3.0 :009 > l = LineItem.first
LineItem Load (0.5ms) SELECT "line_items".* FROM "line_items" ORDER BY "line_items"."id" ASC LIMIT 1
# {
:id => 1,
:order_id => 1,
:created_at => Fri, 22 Jan 2016 14:02:57 UTC +00:00,
:updated_at => Fri, 22 Jan 2016 14:10:19 UTC +00:00
}
2.3.0 :010 > l.order
Order Load (0.4ms) SELECT "orders".* FROM "orders" WHERE "orders"."id" = $1 LIMIT 1 [["id", 1]]
# {
:id => 1,
:customer_id => 1,
:created_at => Fri, 22 Jan 2016 14:07:43 UTC +00:00,
:updated_at => Fri, 22 Jan 2016 14:07:43 UTC +00:00
}
2.3.0 :011 > l.order.customer
Customer Load (0.4ms) SELECT "customers".* FROM "customers" WHERE "customers"."id" = $1 LIMIT 1 [["id", 1]]
# {
:id => 1,
:created_at => Fri, 22 Jan 2016 14:04:22 UTC +00:00,
:updated_at => Fri, 22 Jan 2016 14:04:22 UTC +00:00
}
加入includes之后的sql
l= LineItem.first
LineItem Load (0.7ms) SELECT "line_items".* FROM "line_items" ORDER BY "line_items"."id" ASC LIMIT 1
# {
:id => 1,
:order_id => 1,
:created_at => Fri, 22 Jan 2016 14:02:57 UTC +00:00,
:updated_at => Fri, 22 Jan 2016 14:10:19 UTC +00:00
}
2.3.0 :002 > l.order
Order Load (0.3ms) SELECT "orders".* FROM "orders" WHERE "orders"."id" = $1 LIMIT 1 [["id", 1]]
Customer Load (2.5ms) SELECT "customers".* FROM "customers" WHERE "customers"."id" IN (1)
# {
:id => 1,
:customer_id => 1,
:created_at => Fri, 22 Jan 2016 14:07:43 UTC +00:00,
:updated_at => Fri, 22 Jan 2016 14:07:43 UTC +00:00
}
2.3.0 :003 > l.order.customer
# {
:id => 1,
:created_at => Fri, 22 Jan 2016 14:04:22 UTC +00:00,
:updated_at => Fri, 22 Jan 2016 14:04:22 UTC +00:00
}
查询
N+1 问题
# IN (1,7) 是因为有关联的order的id是 1和7
line_items = LineItem.includes(:order).limit(10)
LineItem Load (0.5ms) SELECT "line_items".* FROM "line_items" LIMIT 10
Order Load (0.3ms) SELECT "orders".* FROM "orders" WHERE "orders"."id" IN (1, 7)
line_items.each do |l| #现在这里不会有sql产生
l.order
end
一次性加载多个关联 (可以看到belongs_to 和 has_many的区别)
Order.includes(:customer, :line_items)
Order Load (0.3ms) SELECT "orders".* FROM "orders"
Customer Load (0.3ms) SELECT "customers".* FROM "customers" WHERE "customers"."id" IN (1, 2)
LineItem Load (0.3ms) SELECT "line_items".* FROM "line_items" WHERE "line_items"."order_id" IN (1, 2, 4, 5, 6, 7, 3)
嵌套关联这里先不多说
includes的过滤（这里会牵扯到join）
- left outer join
Order.includes(:line_items).where("line_items.id" => 5)
SQL (0.5ms) SELECT "orders"."id" AS t0_r0, "orders"."customer_id" AS t0_r1, "orders"."created_at" AS t0_r2, "orders"."updated_at" AS t0_r3, "line_items"."id" AS t1_r0, "line_items"."order_id" AS t1_r1, "line_items"."created_at" AS t1_r2, "line_items"."updated_at" AS t1_r3 FROM "orders"
LEFT OUTER JOIN "line_items" ON "line_items"."order_id" = "orders"."id"
WHERE "line_items"."id" = 5
[
[0] # {
:id => 7,
:customer_id => nil,
:created_at => Fri, 22 Jan 2016 14:28:36 UTC +00:00,
:updated_at => Fri, 22 Jan 2016 14:28:36 UTC +00:00
}
]
- inner join
Order.joins(:line_items).where("line_items.id > ?", 4).group("orders.id")
Order Load (0.6ms) SELECT "orders".* FROM "orders"
INNER JOIN "line_items" ON "line_items"."order_id" = "orders"."id"
WHERE (line_items.id > 4) GROUP BY orders.id
[
[0] # {
:id => 5,
:customer_id => nil,
:created_at => Fri, 22 Jan 2016 14:28:35 UTC +00:00,
:updated_at => Fri, 22 Jan 2016 14:28:35 UTC +00:00
},
[1] # {
:id => 7,
:customer_id => nil,
:created_at => Fri, 22 Jan 2016 14:28:36 UTC +00:00,
:updated_at => Fri, 22 Jan 2016 14:28:36 UTC +00:00
}
]