Rails include only selected columns from relation

Question

I will try to explain:

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.

Here is the code:

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

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)

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

My models:

Product.rb

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

Store.rb:

class Loja < ActiveRecord::Base
  has_many :products
end

Any idea?

Answer 1

Just tried something similar to this on my project. This should work:

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

Using joins as opposed to includes will make rails perform only one query, because joins doesn't eager-load the association.

Answer 2

You can do this via pluck and joins:

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

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.

---

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, ... }]

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