Rails 选择所有列

Question

我有一个 Article 模型 class 与 User 模型

具有一对多连接

class Article < ApplicationRecord
  belongs_to :user
end

class User < ApplicationRecord
  has_many :articles
end

当我运行

article = Article.find(1)
article.user.username

最后一条语句加载整个 table。选择所有列是性能的缺点之一。这是 Rails 控制台输出：

User Load (0.2ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]

但我只想要 username 列。

有没有办法使用相同的表示法 (model_instance.model_instance.column)？

Answer 1

您可以使用两个模型之间的关系并获得所需的用户：

User.select(:name).joins(:articles).find_by(articles: { id: 1 }).username
# SELECT  "users"."username" FROM "users" INNER JOIN "articles" ON "articles"."user_id" = "users"."id" WHERE "articles"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]

也可以写成查询稍作改动：

User.select(:name).joins(:articles).find_by('articles.id = 1').username
# SELECT  "users"."username" FROM "users" INNER JOIN "articles" ON "articles"."user_id" = "users"."id" WHERE (articles.id = 1) LIMIT ?  [["LIMIT", 1]]

或者在使用 where 的情况下，您必须从结果中访问特定记录：

User.select(:name).joins(:articles).where(articles: { id: 1 }).first.username
# SELECT  "users"."username" FROM "users" INNER JOIN "articles" ON "articles"."user_id" = "users"."id" WHERE "articles"."id" = ? ORDER BY "users"."id" ASC LIMIT ?

Rails 选择所有列

Rails selecting all columns

ruby-on-rails

associations

rails-activerecord