In Rails, can I query and only get objects that have a linked object?

Question

So I have this object, let's call it Dog, and this other object, let's call it Collar.

class Dog < ActiveRecord::Base
  has_one :collar
end

class Collar < ActiveRecord::Base
  belongs_to :dog
end

Can I get a list of dogs that have collars? Ideally in a slightly less awkward way than the following:

Dog.joins(:collar).where(Collar.arel_table['id'].not_eq(nil))

As a note to those who might wonder about the "arel_table" bit, it is the DB-agnostic way of saying "IS NOT NULL".

score 1 · Answer 1 · answered Sep 18 '13 at 16:09

1

and what about just create scope in Collar or Dog model something like:

scope :with_something, where( "foreign_key IS NOT NULL" )

answered Sep 18 '13 at 16:09

Jakub Kuchar

That's not a bad idea, but it feels like too much SQL and not enough AREL. `scope :with_collar, ->{ joins(:collar) }` sounds pretty cool though. – Aldric Sep 18 '13 at 16:41

Billy Chan · Accepted Answer · 2013-09-18T16:17:10.713

1

You can just use joins directly

Dog.joins(:collar)

This will return dogs which have collar.

There is even no need to add uniq because this is has_one relation so no duplicate dogs will be returned.

edited Sep 18 '13 at 16:17

answered Sep 18 '13 at 16:11

Billy Chan

2 Answers2