Question

问题

我有以下ActiveRecord模型：

class Person
  belongs_to :favourite_car, class_name: 'Car'
  belongs_to :business_car, class_name: 'Car'
  belongs_to :home_car, class_name: 'Car'
end

当我想访问所有这三个关联时，它会生成三个选择查询：

SELECT * FROM cars WHERE cars.id = ?

这基本上是 N + 1问题。

理想情况下，我希望只生成一个表单

的查询

SELECT * FROM cars WHERE cars.id IN (?, ?, ?)

可能的解决方案

我可以将此移动到与连接表中的列的has_many :through => :join_table关联，以指示关联类型是什么，然后使用includes([:join_table, :cars])来急切加载关联。但是，在这种情况下，它只将3个查询减少到2，并引入了一个额外的表。

另一种可能的解决方案

另一种可能的解决方案是手动加载关联，如下所示：

module EagerLoader
  def eager_load(*associations)
    reflections = associations.map { |association| self.class.reflections[association.to_sym] }
    raise 'Not all are valid associations' if reflections.any?(&:nil?)

    reflections.group_by { |association| association.klass }.each do |klass, reflections|
      load_associations(klass, reflections)
    end

    self

  end

private

  def load_associations(klass, reflections)

    primary_key = klass.primary_key

    ids = reflections.map { |reflection| public_send(reflection.foreign_key) }

    records = klass.where(id: ids)


    reflections.each_with_index do |reflection, i|
      record = records.find do |record|
        record.public_send(primary_key) == ids[i]
      end

      public_send("#{reflection.name}=", record)
    end

  end


end

我已经测试过了，它确实有效。

class Person
  include EagerLoader
end

Person.find(2).eager_load(:favorite_car, :business_car, :home_car)

但是，当您想要执行

之类的操作时，这仍然无法帮助您

Person.includes(:favourite_car, :business_car, :home_car)

例如，在人员索引页面上。这会将查询数量从3N + 1减少到4，但实际上只需要2个。

这个问题有更好的解决方案吗？

Answer 1

试试这个：

> person_id = 1
> person = Person.includes(:favorite_car, :business_car, :home_car).where("people.id" = ?", person_id).references(:favorites_car, :business_car, :home_car)
> person[0].favorite_car

应用/模型/ person.rb

class Person < ActiveRecord::Base # columns: id, name, favorite_car_id, business_car_id, home_car_id belongs_to :favorite_car, class_name: 'Car' belongs_to :business_car, class_name: 'Car' belongs_to :home_car, class_name: 'Car' end

应用/模型/ car.rb

class Car < ActiveRecord::Base # columns: id, name has_many :people end

证明这是有效的：

> Person.all Person Load (0.3ms) SELECT "people".* FROM "people" => #<ActiveRecord::Relation [#<Person id: 1, favorite_car_id: 1, business_car_id: 2, home_car_id: 3, name: "Frankie", created_at: "2014-02-18 21:51:58", updated_at: "2014-02-18 21:53:34">]> > Car.all Car Load (0.3ms) SELECT "cars".* FROM "cars" => #<ActiveRecord::Relation [#<Car id: 1, name: "Mazda", created_at: "2014-02-18 21:52:16", updated_at: "2014-02-18 21:52:16">, #<Car id: 2, name: "Honda", created_at: "2014-02-18 21:52:20", updated_at: "2014-02-18 21:52:20">, #<Car id: 3, name: "BMW", created_at: "2014-02-18 21:52:24", updated_at: "2014-02-18 21:52:24">]> > Person.includes(:favorite_car, :business_car, :home_car).where("people.id = ?", 1).references(:favorite_car, :business_car, :home_car) SQL (0.4ms) SELECT "people"."id" AS t0_r0, "people"."favorite_car_id" AS t0_r1, "people"."business_car_id" AS t0_r2, "people"."home_car_id" AS t0_r3, "people"."name" AS t0_r4, "people"."created_at" AS t0_r5, "people"."updated_at" AS t0_r6, "cars"."id" AS t1_r0, "cars"."name" AS t1_r1, "cars"."created_at" AS t1_r2, "cars"."updated_at" AS t1_r3, "business_cars_people"."id" AS t2_r0, "business_cars_people"."name" AS t2_r1, "business_cars_people"."created_at" AS t2_r2, "business_cars_people"."updated_at" AS t2_r3, "home_cars_people"."id" AS t3_r0, "home_cars_people"."name" AS t3_r1, "home_cars_people"."created_at" AS t3_r2, "home_cars_people"."updated_at" AS t3_r3 FROM "people" LEFT OUTER JOIN "cars" ON "cars"."id" = "people"."favorite_car_id" LEFT OUTER JOIN "cars" "business_cars_people" ON "business_cars_people"."id" = "people"."business_car_id" LEFT OUTER JOIN "cars" "home_cars_people" ON "home_cars_people"."id" = "people"."home_car_id" WHERE (people.id = 1) => #<ActiveRecord::Relation [#<Person id: 1, favorite_car_id: 1, business_car_id: 2, home_car_id: 3, name: "Frankie", created_at: "2014-02-18 21:51:58", updated_at: "2014-02-18 21:53:34">]>

重要说明： Rails会自动使用people作为person的复数形式。因此，当您创建Person模型时，它将创建一个people数据库表。

Answer 2

关于手动预先加载的帖子很棒。

http://mrbrdo.wordpress.com/2013/09/25/manually-preloading-associations-in-rails-using-custom-scopessql/

我想这就是你一直在寻找的东西：

owners = People.all
association_name = :photos

owners.each do |owner|
   record = whatever_you_want

   association = owner.association(association_name)
   association.target = record
   association.set_inverse_instance(record)
end

Answer 3

我个人创建了一个连接表。话虽如此，这是另一种选择：可以基于SQL查询定义关系：

class Person
  belongs_to :favourite_car, class_name: 'Car'
  belongs_to :business_car, class_name: 'Car'
  belongs_to :home_car, class_name: 'Car'

  has_many :cars, class_name: 'Car', :finder_sql => %q(
   SELECT DISTINCT cars.*
   FROM cars
   WHERE cars.id IN (#{c_ids})
  )

  def c_ids
    [favourite_car_id, business_car_id, home_car_id].compact.uniq.join(',')
  end
end

然后你可以做Person.includes（：cars）

ActiveRecord渴望加载多个belongs_to关联

问题

可能的解决方案

另一种可能的解决方案

3 个答案: