Question

I have an SQL query that returns what I need, but I'm having trouble converting this into an active record query the "Rails way".

My SQL query is:

SELECT * from trips 
WHERE trips.title LIKE "%Thailand%"
AND EXISTS (SELECT * from places WHERE places.trip_id = trips.id AND places.name LIKE "%Bangkok%")
AND EXISTS (SELECT * from places WHERE places.trip_id = trips.id AND places.name LIKE "%Phuket%")

I'm trying something like this using Rails:

@trips=Trip.where("trips.title LIKE ?", "%Thailand%")
@trips=@trips.includes(:places).where(("places.name LIKE ?","%Bangkok%").exists?) => true, ("places.name LIKE ?","%Phuket%").exists?) => true)

But it doesn't seem to work and i'm stumped as to what to try.

Answer 1

使用Where Exists gem（公平说明：我是作者）：

Trip.where("trips.title LIKE ?", "%Thailand%")
  .where_exists(:places, ['name LIKE ?', '%Bangkok%'])
  .where_exists(:places, ['name LIKE ?', '%Phuket%'])

Answer 2

这可以做到，但它有点不可思议：

Trip.where("trips.title LIKE ?", "%Thailand%")
    .joins(:places)
    .where( Place.where('name LIKE ?', '%Bangkok%').exists )
    .where( Place.where('name LIKE ?', '%Phuket%').exists )

我说＆＃34; wonky＆＃34;因为您的where条件只能包含文字字符串;使用像where(name: 'Phuket')这样的哈希语法会使Rails barf变得奇怪：

> User.joins(:groups).where( Group.where(name: 'a').exists )
User Load (3.2ms)  SELECT "users".* FROM "users" INNER JOIN "groups" ON "groups"."user_id" = "users"."id" WHERE (EXISTS (SELECT "groups".* FROM "groups" WHERE "groups"."name" = $1))
PG::ProtocolViolation: ERROR:  bind message supplies 0 parameters, but prepared statement "" requires 1
: SELECT "users".* FROM "users" INNER JOIN "groups" ON "groups"."user_id" = "users"."id" WHERE (EXISTS (SELECT "groups".* FROM "groups" WHERE "groups"."name" = $1))
ActiveRecord::StatementInvalid: PG::ProtocolViolation: ERROR:  bind message supplies 0 parameters, but prepared statement "" requires 1
: SELECT "users".* FROM "users" INNER JOIN "groups" ON "groups"."user_id" = "users"."id" WHERE (EXISTS (SELECT "groups".* FROM "groups" WHERE "groups"."name" = $1))

该错误来自我所处的随机数据模型，但我预计会出现同样的错误。不幸的是，Rails在处理嵌套查询时搞砸了它如何绑定参数。

另一种方法是使用.where('EXISTS(SELECT 1 FROM places WHERE name LIKE ?', '%Bangkok%')，直接将SQL嵌入到您的应用程序中。它不像时髦或酷，但从长远来看，它可能更容易维护！

Answer 3

这是我第一次尝试的重构版本，其结果更接近您的预期目标（我对第一次，误导性的目标道歉）。

在app/models/trip.rb：

scope :with_title, ->(title) { where(arel_table[:title].matches('%' + title + '%')) }

def self.has_place(place_name)
  _trips = Trip.arel_table
  _places = Place.arel_table
  where(Place.joins(:trip).where(_places[:name].eq(place_name)).exists)
end

然后你可以写：

Trip.for_title('Thailand').has_place('Bangkok').has_place('Phuket')

将提供以下SQL：

SELECT "trips".* FROM "trips" WHERE ("trips"."title" LIKE '%Thailand%') AND (EXISTS (SELECT "places".* FROM "places" INNER JOIN "trips" ON "trips"."id" = "places"."trip_id" WHERE "places"."name" = 'Bangkok')) AND (EXISTS (SELECT "places".* FROM "places" INNER JOIN "trips" ON "trips"."id" = "places"."trip_id" WHERE "places"."name" = 'Phuket'))

这使您可以灵活地将查询拼凑在一起，而无需在查询中依赖硬编码SQL，并且可以在数据库引擎之间移植。

此解决方案假定放置belongs_to Trip（及其补码）。如果关联是/（不同），请调整joins子句。

此解决方案的灵感部分来自How to do "where exists" in Arel和Make search NOT case sensitive on my rails app。

Answer 4

不使用gem可以更容易出错。当前接受的答案（在撰写本文时）是手动完成的，实际上并未生成正确的SQL。（此问题已得到修复，但这突出了出错的风险。）

另一个可以做到这一点的宝石：activerecord_where_assoc（我是作者）

有了它，您可以按照以下方式做您想做的事情：

Trip.where("trips.title LIKE ?", "%Thailand%")
    .where_assoc_exists(:places, ['name LIKE ?', '%Bangkok%'])
    .where_assoc_exists(:places, ['name LIKE ?', '%Phuket%'])

activerecord_where_assoc具有full documentation的强大功能，并且针对Rails 4.2到6.0进行了CI测试。这是introduction。

Rails ActiveRecord WHERE EXISTS query

4 个答案: