我有一个Article
模型类,其中包含与User
模型的一对多连接
class Article < ApplicationRecord
belongs_to :user
end
class User < ApplicationRecord
has_many :articles
end
当我跑步时
article = Article.find(1)
article.user.username
最后一个语句加载整个表。选择所有列是表演的邪恶之一。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符号(model_instance.model_instance.column
)?
答案 0 :(得分:3)
您可以使用两个模型之间的关系并获取所需的用户:
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]]
它也可以写成查询略有变化:
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]]
或者如果使用where
,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 ?