ActiveRecord provides an easy way to select specific fields from the table
using the ActiveRecord::QueryMethods#select method.
Recently ActiveRecord::QueryMethods#select
added support
for hash values.
Let’s see some examples with the Product
and
Variant models
class Product < ApplicationRecord
# id, name, description
has_many :variants
end
class Variant < ApplicationRecord
# id, price, quantity, product_id
belongs_to :product
endBefore
Before 7.1, we were able to select specific fields with the following syntax:
Product
.joins(:variants)
.select("products.id, products.name, variants.price")
# OR
Product
.joins(:variants)
.select(:id, :name, "variants.price")The above results in:
SELECT products.id, products.name, variants.price
FROM "products"
INNER JOIN "variants" ON "variants"."product_id" = "products"."id"
LIMIT $1 [["LIMIT", 11]]
/* OR */
SELECT "products"."id", "products"."name", "variants"."price"
FROM "products"
INNER JOIN "variants" ON "variants"."product_id" = "products"."id"
LIMIT $1 [["LIMIT", 11]]After
After ActiveRecord::QueryMethods#select added support for hash we don’t need to use the raw version of the query anymore.
Let’s take a look at the same code after this change.
Product
.joins(:variants)
.select(:id, :name, variants: [:price])The above will result in:
SELECT "products"."id", "products"."name", "variants"."price"
FROM "products"
INNER JOIN "variants" ON "variants"."product_id" = "products"."id"
LIMIT $1 [["LIMIT", 11]]We can also use the following syntax that supports alias:
Product
.joins(:variants)
.select(
products: { id: :product_id, name: :product_name },
variants: { price: :product_price }
)The above will result in:
SELECT "products"."id" AS product_id, "products"."name" AS product_name, "variants"."price" AS product_price
FROM "products"
INNER JOIN "variants" ON "variants"."product_id" = "products"."id"
LIMIT $1 [["LIMIT", 11]]
