Is it possible to ask for only certain columns from an ActiveRecord association?

Question

consider

def Foo
  has_one :user
end

let's say i only want a Foo's User's name, and not any of the other columns. so i want

SELECT name FROM "users" WHERE "prices"."id" = 123

but doing foo.user.name will give me

SELECT * FROM "users" WHERE "prices"."id" = 123

is there any slick way to use the association to get only one column? if not, then i have to do:

User.where(id: foo.user_id).pluck(:name).first

Answer 1

In general you can specify what columns you want to select using the .select method, like:

User.select(:name).where(...)

This will return just the values from the name column. You can chain this onto an association, but not onto an instance. So, as meagar very agressively pointed out by downvoting the other answers (including Mori's deleted answer), in a has_one relationship you can't chain this on the association (because it's not an association in that case). However, you could build a custom scope, like this:

class Foo < ActiveRecord::Base
  has_one :bar
  scope :bar_name, lambda {Bar.select(:name).where(:foo_id=> id)}
end

The above is untested so you may have to tweak it, but generally speaking that approach would allow you to do something like:

foo.bar_name

...without loading all the columns from Bar.

Answer 2

No, in the case of your has_one, but yes in the case of has_many.

The object returned for a has_one association isn't a scope onto which you can chain additional methods like select, like with a has_many. It's an actual instance of the model, and instantiating it will necessarily involve a select *.

If you want to select just the name, you'll have to access the User model directly and use select.

Conversely, if your Foo has many users, you could use foo.users.select("name") or any of the other chainable methods, as foo.users would be an actual ActiveRecord association, not an instance of a model.