sql – 在ActiveRecord / Arel(Rails 4)中进行联合交集的有效方法？

2019年8月4日 200次阅读

我正在尝试构建一个非常复杂的ActiveRecord查询,涉及两个联合的交集.我的解决方案有效,但生成的SQL中有很多冗余.有更有效的方法吗？

在我的宇宙中有人,猫,狗,书和专辑. (这是实际模型设置的玩具版本,它涉及更复杂的关联,因此甚至更丑陋的SQL.)

class Person < ActiveRecord::Base
  has_many :cats
  has_many :dogs
  has_many :books
  has_many :albums

  ...
end

我想要找回所有拥有名为琼斯的人,他们拥有一只名叫华莱士的宠物,以及一本1996年发行的书籍或专辑.我希望能够构建如下的查询：

Person.named("Jones").with_pet_named("Wallace").with_book_or_album_released_in(1996)

“命名琼斯”部分很简单：

scope :named, ->(name) { where(name: name) }

查询的另外两部分比较棘手.我现在正在这样做的方式,我正在使用Arel方法来构建联合(使用从这个谈话借来的模式：http://danshultz.github.io/talks/mastering_activerecord_arel/#/)：

class Person
  ...

  def self.with_pet_named(name)
    with_cat_named = joins(:cats).where(cats: {name: name})
    with_dog_named = joins(:dogs).where(dogs: {name: name})
    with_cat_or_dog_named = with_cat_named.union(with_dog_named)
    from(arel_table.create_table_alias(with_cat_or_dog_named, :people))
  end

  def self.with_book_or_album_released_in(year)
    with_book_released_in = joins(:books).where(books: {release_date: year})
    with_album_released_in = joins(:albums).where(albums: {release_date: year})
    with_book_or_album_released_in = with_book_released_in.union(with_album_released_in)
    from(arel_table.create_table_alias(with_book_or_album_released_in, :people))
  end
end

当我将这些范围和方法链接在一起时,问题就出现了.检索到正确的人,但SQL看起来不是很好.

Person.named("Jones").with_pet_named("Wallace").with_book_or_album_released_in(1996).
  to_sql

回报

SELECT "people".* FROM
  ( SELECT "people".* FROM
    ( SELECT "people".* FROM "people"
        INNER JOIN "cats"
        ON "cats"."person_id" = "people"."id"
        WHERE "people"."name" = 'Jones' AND "cats"."name" = 'Wallace'
      UNION
      SELECT "people".* FROM "people"
        INNER JOIN "dogs"
        ON "dogs"."person_id" = "people"."id"
        WHERE "people"."name" = 'Jones' AND "dogs"."name" = 'Wallace'
    ) "people"
      INNER JOIN "books"
      ON "books"."person_id" = "people"."id"
      WHERE "people"."name" = 'Jones' AND "book"."release_date" = 1996
    UNION
    SELECT "people".* FROM
    ( SELECT "people".* FROM "people"
        INNER JOIN "cats"
        ON "cats"."person_id" = "people"."id"
        WHERE "people"."name" = 'Jones' AND "cats"."name" = 'Wallace'
      UNION
      SELECT "people".* FROM "people"
        INNER JOIN "dogs"
        ON "dogs"."person_id" = "people"."id"
        WHERE "people"."name" = 'Jones' AND "dogs"."name" = 'Wallace'
    ) "people"
      INNER JOIN "albums"
      ON "albums"."person_id" = "people"."id"
      WHERE "people"."name" = 'Jones' AND "album"."release_date" = 1996
  ) "people"
  WHERE "people"."name" = 'Jones'

我想我想要更像这样的东西：

SELECT "people".* FROM
  ( ( SELECT "people".* FROM "people"
        INNER JOIN "cats"
        ON "cats"."person_id" = "people"."id"
        WHERE "cats"."name" = 'Wallace'
      UNION
      SELECT "people".* FROM "people"
        INNER JOIN "dogs"
        ON "dogs"."person_id" = "people"."id"
        WHERE "dogs"."name" = 'Wallace'
    ) INTERSECT
    ( SELECT "people".* FROM "people"
        INNER JOIN "books"
        ON "books"."person_id" = "people"."id"
        WHERE "book"."release_date" = 1996
      UNION
      SELECT "people".* FROM "people"
        INNER JOIN "albums"
        ON "albums"."person_id" = "people"."id"
        WHERE "album"."release_date" = 1996
    )
  ) "people"
  WHERE "people"."name" = 'Jones'

虽然笨拙,但至少不是多余的.有没有办法使用ActiveRecord和Arel方法获得更简洁的SQL查询,而无需更改我的模型或关联？

最佳答案 没有你的数据库架构,很难测试这个,但我认为这应该让你更接近你想要的SQL.

注意：此“解决方案”会破坏代码中可用的方法链.

class Person
  #…
  def self.with_pet_named(name)
    with_cat_named = joins(:cats).where(cats: {name: name})
    with_dog_named = joins(:dogs).where(dogs: {name: name})
    Arel::Nodes::Union.new(with_cat_named.ast, with_dog_named.ast)
  end

  def self.with_book_or_album_released_in(year)
    with_book_released_in = joins(:books).where(books: {release_date: year})
    with_album_released_in = joins(:albums).where(albums: {release_date: year})
    Arel::Nodes::Union.new(with_book_released_in.ast, with_album_released_in.ast)
  end
  #…
end

inter = Arel::Nodes::Intersect.new(
  Person.with_pet_named('Wallace'),
  Person.with_book_or_album_released_in(1996)
)
p = Arel::Nodes::TableAlias.new(inter, :p)
Person.from(p).where(p[:name].eq('Jones')).to_sql
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