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schema.rb
ActiveRecord::Schema.define(version: 20150203032005) do create_table "addresses", force: true do |t| t.integer "client_id" t.string "street" t.string "postcode" t.datetime "created_at" t.datetime "updated_at" end create_table "clients", force: true do |t| t.string "name" t.string "gender" t.datetime "created_at" t.datetime "updated_at" end create_table "infos", force: true do |t| t.integer "address_id" t.string "history" t.datetime "created_at" t.datetime "updated_at" end end
client.rb
class Client < ActiveRecord::Base has_one :address end
address.rb
class Address < ActiveRecord::Base belongs_to :client has_many :infos end
info.rb
class Address < ActiveRecord::Base belongs_to :client has_many :infos end
Client.all
Client Load (0.4ms) SELECT "clients".* FROM "clients" => #<ActiveRecord::Relation [#<Client id: 1, name: "wei", gender: "1", created_at: "2015-02-03 02:12:36", updated_at: "2015-02-03 02:12:36">, #<Client id: 2, name: "yan", gender: "1", created_at: "2015-02-03 02:48:11", updated_at: "2015-02-03 02:48:11">]>
Address.all
SELECT "addresses".* FROM "addresses" => #<ActiveRecord::Relation [#<Address id: 1, client_id: 1, street: "huanyuan", postcode: "123456", created_at: "2015-02-03 02:13:45", updated_at: "2015-02-03 02:13:45">, #<Address id: 2, client_id: 2, street: "huayuan", postcode: "23456", created_at: "2015-02-03 02:50:12", updated_at: "2015-02-03 02:50:12">]>
Info.all
SELECT "infos".* FROM "infos" => #<ActiveRecord::Relation
[#<Info id: 1, address_id: 1, history: "1110", created_at: "2015-02-03 03:25:34", updated_at: "2015-02-03 03:25:34">, #<Info id: 2, address_id: 1, history: "1111", created_at: "2015-02-03 03:26:16", updated_at: "2015-02-03 03:26:16">, #<Info id: 3, address_id: 1, history: "1112", created_at: "2015-02-03 03:26:21", updated_at: "2015-02-03 03:26:21">, #<Info id: 4, address_id: 2, history: "2110", created_at: "2015-02-03 03:26:32", updated_at: "2015-02-03 03:26:32">, #<Info id: 5, address_id: 2, history: "2111", created_at: "2015-02-03 03:26:34", updated_at: "2015-02-03 03:26:34">, #<Info id: 6, address_id: 2, history: "2112", created_at: "2015-02-03 03:26:37", updated_at: "2015-02-03 03:26:37">, #<Info id: 7, address_id: 2, history: "2113", created_at: "2015-02-03 03:26:39", updated_at: "2015-02-03 03:26:39">]>
N + 1 查询的解决办法
clients = Client.limit(2)
clients.each do |client|
puts client.address.postcode
end
生成的sql语句为:
SELECT "clients".* FROM "clients"
=> #<ActiveRecord::Relation
[#<Client id: 1, name: "wei", gender: "1", created_at: "2015-02-03 02:12:36", updated_at: "2015-02-03 02:12:36">,
#<Client id: 2, name: "yan", gender: "1", created_at: "2015-02-03 02:48:11", updated_at: "2015-02-03 02:48:11">]>
而
clients = Client.includes(:address).limit(2)
clients.each do |client|
puts client.address.postcode
end
生成的sql语句为:
Client Load (0.5ms) SELECT "clients".* FROM "clients" LIMIT 2
Address Load (0.4ms) SELECT "addresses".* FROM "addresses" WHERE "addresses"."client_id" IN (1, 2)
=> #<ActiveRecord::Relation [#<Client id: 1, name: "wei", gender: "1", created_at: "2015-02-03 02:12:36", updated_at: "2015-02-03 02:12:36">,
#<Client id: 2, name: "yan", gender: "1", created_at: "2015-02-03 02:48:11", updated_at: "2015-02-03 02:48:11">]>
clients = Client.all clients.each do |client| puts client.address.infos.first.history end
生成的sql语句:
Address Load (0.3ms) SELECT "addresses".* FROM "addresses" WHERE "addresses"."client_id" = ? LIMIT 1 [["client_id", 1]] Info Load (0.3ms) SELECT "infos".* FROM "infos" WHERE "infos"."address_id" = ? ORDER BY "infos"."id" ASC LIMIT 1 [["address_id", 1]] 1110 Address Load (0.1ms) SELECT "addresses".* FROM "addresses" WHERE "addresses"."client_id" = ? LIMIT 1 [["client_id", 2]] Info Load (0.1ms) SELECT "infos".* FROM "infos" WHERE "infos"."address_id" = ? ORDER BY "infos"."id" ASC LIMIT 1 [["address_id", 2]] 2110 => [#<Client id: 1, name: "wei", gender: "1", created_at: "2015-02-03 02:12:36", updated_at: "2015-02-03 02:12:36">, #<Client id: 2, name: "yan", gender: "1", created_at: "2015-02-03 02:48:11", updated_at: "2015-02-03 02:48:11">]
Client和Address是has_one关系， Address和info是has_many关系， 想要一次性加载资源使client可以通过address获取info, 使用下面语句:
clients = Client.includes(address: :infos)
生成的sql语句：
Client Load (0.4ms) SELECT "clients".* FROM "clients" Address Load (0.5ms) SELECT "addresses".* FROM "addresses" WHERE "addresses"."client_id" IN (1, 2) Info Load (0.3ms) SELECT "infos".* FROM "infos" WHERE "infos"."address_id" IN (1, 2) => #<ActiveRecord::Relation [#<Client id: 1, name: "wei", gender: "1", created_at: "2015-02-03 02:12:36", updated_at: "2015-02-03 02:12:36">, #<Client id: 2, name: "yan", gender: "1", created_at: "2015-02-03 02:48:11", updated_at: "2015-02-03 02:48:11">]>
clients.each do |c|
puts c.address.infos.first.history
end
输出:
1110
2110
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原文地址：http://www.cnblogs.com/tardis/p/4269442.html
