ActiveRecord’s Queries tricks 小记

Published 05-04-2019 16:32:44

原文 https://medium.com/rubyinside/active-records-queries-tricks-2546181a98dd

关联表join时使用条件

# User model
scope :activated, ->{
  joins(:profile).where(profiles: { activated: true })
}

更好的做法

# Profile model
scope :activated, ->{ where(activated: true) }
# User model
scope :activated, ->{ joins(:profile).merge(Profile.activated) }

关于 merge https://apidock.com/rails/ActiveRecord/SpawnMethods/merge https://api.rubyonrails.org/classes/ActiveRecord/SpawnMethods.html

嵌套join的差异

  • User has_one Profile
  • Profile has_many Skills

    User.joins(:profiles).merge(Profile.joins(:skills))
    => SELECT users.* FROM users
    INNER JOIN profiles    ON profiles.user_id  = users.id
    LEFT OUTER JOIN skills ON skills.profile_id = profiles.id
    # So you'd rather use:
    User.joins(profiles: :skills)
    => SELECT users.* FROM users 
    INNER JOIN profiles ON profiles.user_id  = users.id
    INNER JOIN skills   ON skills.profile_id = profiles.id
    
    

内链接和外连接

Exist query

存在和不存在

# Post
scope :famous, ->{ where("view_count > ?", 1_000) }
# User
scope :without_famous_post, ->{
  where(_not_exists(Post.where("posts.user_id = users.id").famous))
}
def self._not_exists(scope)
  "NOT #{_exists(scope)}"
end
def self._exists(scope)
  "EXISTS(#{scope.to_sql})"
end

Subqueries 子查询

比如查询部分用户(user)的帖子(post)

不好的做法

Post.where(user_id: User.created_last_month.pluck(:id))

这里的缺陷是将运行两个SQL查询:一个用于获取用户的ID,另一个用于从这些user_id获取帖子

这样写一个查询就可以了

Post.where(user_id: User.created_last_month)

基础

.to_sql 生成 SQL 语句字符串 .explain 获取查询分析

Booleans

对于User.where.not(tall: true)在pg下会生成 SELECT users.* FROM users WHERE users.tall <> 't' 这返回 tall 是 false 的 记录,不包括是null 的

包括null应该这么写

User.where("users.tall IS NOT TRUE")

or

User.where(tall: [false, nil])