Selecting unique rows when doing a join

Question

Fist a little background: I'm using Rails 3.0.7, Postgres 9.0 and MetaWhere/MetaSearch 1.0.4

I have a very frustrating problem with getting the unique records when querying the my database. I have 3 models as follows:

class Car < ActiveRecord::Base
  has_many    :sellers, :through => :adverts, :uniq => true
  has_many    :adverts, :dependent => :destroy
end

class Seller < ActiveRecord::Base
  has_many :cars, :through => :adverts, :uniq => true
  has_many :adverts
end

class Advert < ActiveRecord::Base
  belongs_to :car, :autosave => false
  belongs_to :seller, :autosave => false
end

So far so good. Now what I want to do is, to find all cars, that is Fiat Panda´s (:brand, :model_name attributes on car). This is going just fine. But if I want some information from sellers table joined in as well, the problems start to show - I'm getting duplicate cars!!! What I do is the following:

Car.includes(:adverts, :sellers).where(:brand >> 'Fiat', :model_name >> 'Panda', :sellers => [:kind >> 'Dealer'])

Now you could argue, that this is not possible, because "how should the db know which :kind to chose from all the sellers connected to each car?" But I don't care, because they are all the same, so it doesn't matter if it's the first or if it's the last seller it's taking the attributes value from. If I do a .debug_sql, I get the following:

SELECT cars.*, sellers.*, adverts.* FROM cars LEFT OUTER JOIN adverts ON adverts.car_id = cars.id LEFT OUTER JOIN adverts sellers_cars_join ON cars.id = sellers_cars_join.car_id LEFT OUTER JOIN sellers ON sellers.id = sellers_cars_join.seller_id WHERE cars.brand = 'Fiat' AND cars.model_name = 'Panda' AND sellers.kind = 'Dealer'

Of cause this gives me duplicates, it gives perfect sense - but how can I solve it? - cause it's not what I want.

I can see two possible solutions to this: First If I could somehow in a rails-like way get it to execute

SELECT DISTINCT(cars.id), cars.*, sellers.*, adverts.* FROM cars LEFT....

It seems that it would give me the right thing.

Second As you can see, I have added a :uniq => true to the associations, but this would, as far as I can see only work in my example, if I stated from sellers and asked for cars like this:

Seller.includes(:adverts, :cars).where(:cars => [:brand >> 'Fiat'], :cars => [:model_name >> 'Panda'], :kind >> 'Dealer')

But I'm not sure about this at all! And what about metawhere/metasearch - I'm fearing that it's interfering in this as well.

Answer 1

includes performs a LEFT OUTER JOIN, which indeed creates duplicates. If you don't need to access to each @car.seller after your query (n+1 query issue), just use joins instead :

Car.joins(:sellers).
    where(:cars => {:brand => 'Fiat', 
                    :model_name => 'Panda'}, 
          :sellers => {:kind => 'Dealer'})

joins performs an INNER JOIN, so you shouldn't get duplicates.