Question

OK, I recently had great answers to a question about how to do some sub-selects in an activerecord query. I now have a more complicated situation that I can’t figure out.

I want to do a search across 3 tables that have a many-to-one relationship with a :through , eg

class User << ActiveRecord::Base
  has_many :pets
  has_many :parasites, :through => :pets
end

class Pet << ActiveRecord::Base
  has_many :parasites
  belongs_to :users
end

class Parasite << ActiveRecord::Base
  belongs_to :pets
end

Now let’s say I have some data like so

users

id        name
1         Bob
2         Joe
3         Brian

pets

id        user_id  animal
1         1        cat
2         1        dog
3         2        cat
4         3        dog

parasites

id        pet_id    bug
1         1         tick
2         2         flea
3         3         tick
4         3         flea
5         4         tick

What I want to do is create an active record query that will return a user that has a pet which has both ticks and fleas (i.e. User 2 – Joe)

This is so far beyond my activerecord and sql skills that I won’t even bother to show you my bungled attempts so far.

Answer 1

This is pretty much the same as the previous question, you only need to dig one level deeper on the sub-selects:

User.where('id IN (SELECT user_id FROM pets WHERE
  id IN (SELECT pet_id FROM parasites WHERE bug = ?) AND
  id IN (SELECT pet_id FROM parasites WHERE bug = ?))', 'flea', 'tick')