Cache related ActiveRecord objects for collection

Question

I'd like to select a collection of ActiveRecord objects and their belongs_to related object with a single query done. I.e. after calling

products = Product.joins(:image).limit(10)

and while iterating over products, I want that product.image didn't cause another query to the DB, but it had that image attribute already populated with Image instance cached from previous query instead.

How do I construct params for select, so that it selected all attributes needed to populate related objects? Or maybe I am looking in a wrong direction and there's some other way to reduce these unnecessary queries on each iteration:

Image Load (1.0ms)  SELECT "images".* FROM "images" WHERE "images"."id" = 270783 LIMIT 1
Image Load (1.2ms)  SELECT "images".* FROM "images" WHERE "images"."id" = 121344 LIMIT 1
Image Load (1.2ms)  SELECT "images".* FROM "images" WHERE "images"."id" = 196239 LIMIT 1
Image Load (1.2ms)  SELECT "images".* FROM "images" WHERE "images"."id" = 91194 LIMIT 1 
Image Load (1.2ms)  SELECT "images".* FROM "images" WHERE "images"."id" = 116278 LIMIT 1
Image Load (1.2ms)  SELECT "images".* FROM "images" WHERE "images"."id" = 117207 LIMIT 1
Image Load (0.8ms)  SELECT "images".* FROM "images" WHERE "images"."id" = 244471 LIMIT 1
Image Load (0.8ms)  SELECT "images".* FROM "images" WHERE "images"."id" = 239287 LIMIT 1
Image Load (0.8ms)  SELECT "images".* FROM "images" WHERE "images"."id" = 15124 LIMIT 1 
Image Load (1.2ms)  SELECT "images".* FROM "images" WHERE "images"."id" = 186670 LIMIT 1

Answer 1

What you want is called "eager loading". And all you have to do is change your .joins(:image) to .includes(:image) to get eager loading.

products = Product.includes(:image).limit(10)

See also the Rails Guide on Eager Loading.