Rails仅包含关联中的选定列 [英] Rails include only selected columns from relation

问题描述

我将尝试解释:

我有一个表产品"和一个表商店".我试图只让商店名称显示在页面上，但ActiveRecord会向我返回商店中的所有列.

I have a table 'Product' and a table 'Store'. I'm trying to get just the store name to show at the page, but ActiveRecord is returning me all the columns from the store.

这是代码:

@product = Product
          .order(id: :desc)
          .includes(:store)
          .select("products.id, products.store_id, stores.name")
          .limit(1)

内置查询(来自Rails):

The built query (from Rails):

Processing by IndexController#index as HTML
Product Load (0.0ms)  SELECT  products.id, products.store_id FROM `products`  ORDER BY `products`.`id` DESC LIMIT 1
Store Load (1.0ms)  SELECT `stores`.* FROM `stores` WHERE `stores`.`id` IN (111)

我只想显示产品的商店名称，但是Rails选择了所有商店列.

I just want to show the product's store name, but Rails is selecting all the store columns.

我的模特:

Product.rb

Product.rb

class Product < ActiveRecord::Base
  belongs_to :store, :foreign_key => 'store_id'
end

Store.rb:

class Loja < ActiveRecord::Base
  has_many :products
end

有什么主意吗?

推荐答案

您可以通过采摘和加入来做到这一点:

You can do this via pluck and joins:

Product
  .order(id: :desc)
  .joins(:store)
  .pluck("products.id, products.store_id, stores.name")
  .limit(1)

您将获得一个数组，该数组遵循pluck中定义的列的顺序.如果您不熟悉拔毛，我已经写过有关拔毛的文章.

You're going to get an array back that follows the order of the columns defined in pluck. I've written about pluck before if you're not familiar with it.

另一种方法是通过直接的SQL查询:

The other way is via a straight SQL query:

sql =<<-EOF
  SELECT products.id, products.store_id, stores.name
  FROM products INNER JOIN stores
    ON products.store_id = stores.id
  ORDER BY products.id DESC
  LIMIT 1
EOF
records = ActiveRecord::Base.connection.execute(sql)

#=> [{"id" => X, "store_id" => Y, "name" => Z, ... }]

两个选项都产生相同的SQL语句，只是结果的显示方式有所不同.

Both options produce the same SQL statement, the just vary in how the results come out.

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