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Rails 提供了四种不同加载关联数据的方法。下面就来介绍一下。
Preload 是以附加一条查询语句来加载关联数据的
1 User.preload(:posts).to_a 2 3 # => 4 SELECT "users".* FROM "users" 5 SELECT "posts".* FROM "posts" WHERE "posts"."user_id" IN (1)
这种方式与是 includes 默认加载数据的方式
由于 preload 总是生成两条 sql，所以不能在后面使用 where 条件，下面的查询会报错
User.preload(:posts).where("posts.desc=‘ruby is awesome‘") # => SQLite3::SQLException: no such column: posts.desc: SELECT "users".* FROM "users" WHERE (posts.desc=‘ruby is awesome‘)
在 preload 的 where 条件只能这样使用
User.preload(:posts).where("users.name=‘Neeraj‘") # => SELECT "users".* FROM "users" WHERE (users.name=‘Neeraj‘) SELECT "posts".* FROM "posts" WHERE "posts"."user_id" IN (3)
默认情况下 Includes 加载关联数据和 preload 一样
但它比 preload 要聪明一些。上面看到关于 preload 的查询 User.preload(:posts).where("posts.desc=‘ruby is awesome‘") 会失败。下面看看 includes：
User.includes(:posts).where(‘posts.desc = "ruby is awesome"‘).to_a # => SELECT "users"."id" AS t0_r0, "users"."name" AS t0_r1, "posts"."id" AS t1_r0, "posts"."title" AS t1_r1, "posts"."user_id" AS t1_r2, "posts"."desc" AS t1_r3 FROM "users" LEFT OUTER JOIN "posts" ON "posts"."user_id" = "users"."id" WHERE (posts.desc = "ruby is awesome")
这里你可以看到 includes 把两条分开的 SQL 用 LEFT OUTER JOIN 生成一条数据，并且它还包括了 where 条件
所以在一些情况下 includes 把两条语句合成一条。但最简单情况下它是使用两条的。但如果你就想要 includes 使用一条 sql 语句, references 可以做到：
User.includes(:posts).references(:posts).to_a # => SELECT "users"."id" AS t0_r0, "users"."name" AS t0_r1, "posts"."id" AS t1_r0, "posts"."title" AS t1_r1, "posts"."user_id" AS t1_r2, "posts"."desc" AS t1_r3 FROM "users" LEFT OUTER JOIN "posts" ON "posts"."user_id" = "users"."id"
上面只会生成一条 sql
eager loading 是以 LEFT OUTER JOIN 加载所有相关数据的。
User.eager_load(:posts).to_a # => SELECT "users"."id" AS t0_r0, "users"."name" AS t0_r1, "posts"."id" AS t1_r0, "posts"."title" AS t1_r1, "posts"."user_id" AS t1_r2, "posts"."desc" AS t1_r3 FROM "users" LEFT OUTER JOIN "posts" ON "posts"."user_id" = "users"."id"
它正是 includes 使用 where 或 order 从 posts表中获取数据时强制生成一条 sql 的情况。
Joins 方式是使用 inner join 加载关联数据
User.joins(:posts) # => SELECT "users".* FROM "users" INNER JOIN "posts" ON "posts"."user_id" = "users"."id"
上面的情况下 posts 数据不会被查询出来，它会得到重复数据。举个例子：
def self.setup User.delete_all Post.delete_all u = User.create name: ‘Neeraj‘ u.posts.create! title: ‘ruby‘, desc: ‘ruby is awesome‘ u.posts.create! title: ‘rails‘, desc: ‘rails is awesome‘ u.posts.create! title: ‘JavaScript‘, desc: ‘JavaScript is awesome‘ u = User.create name: ‘Neil‘ u.posts.create! title: ‘JavaScript‘, desc: ‘Javascript is awesome‘ u = User.create name: ‘Trisha‘ end
上面的测试数据运行之后会得到：
#<User id: 9, name: "Neeraj"> #<User id: 9, name: "Neeraj"> #<User id: 9, name: "Neeraj"> #<User id: 10, name: "Neil">
我们可以使用 distinct 来去重：
User.joins(:posts).select(‘distinct users.*‘).to_a
如果想要 posts 表的数据也可以 select 出来
records = User.joins(:posts).select(‘distinct users.*, posts.title as posts_title‘).to_a records.each do |user| puts user.name puts user.posts_title end
注意：在应用 joins 查询数据的时候使用 user.posts 会生成另外一个 sql 语句
原文地址：
http://blog.bigbinary.com/2013/07/01/preload-vs-eager-load-vs-joins-vs-includes.html
rails 中 preload、includes、Eager load、Joins 的区别
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原文地址：http://www.cnblogs.com/mikedeng/p/difference_of_preload_and_includes_joins.html
