Why do I get duplicate results using scope on Heroku, but not on my local machine?

I have this scope in Subscriber.rb:

scope :notify_today, 
joins(:skills => :positions).
where('positions.created_at > subscribers.created_at AND positions.created_at > ?', 1.day.ago)

On my local machine it works fine, but on Heroku it returns each object twice.

I managed to fix it with a g开发者_StackOverflow中文版roup:

scope :notify_today, 
joins(:skills => :positions).
where('positions.created_at > subscribers.created_at AND positions.created_at > ?', 1.day.ago).
group('subscribers.email, subscribers.id, subscribers.last_email_at, subscribers.created_at, subscribers.updated_at')

I just wondered if this is the right way to fix it and why Heroku duplicates the results?

EDIT:

Database associations:

Subscriber
has_and_belongs_to_many :skills

Skill
has_and_belongs_to_many :subscribers
has_many :positions

Position
belongs_to :skill

I'm using the default database that comes with Rails.

Subscriber.notify_today.to_sql gives the same thing on my local machine and on heroku:

   "SELECT \"subscribers\".* 
    FROM \"subscribers\" 
    INNER JOIN \"skills_subscribers\" ON \"skills_subscribers\".\"subscriber_id\" = \"subscribers\".\"id\" 
    INNER JOIN \"skills\" ON \"skills\".\"id\" = \"skills_subscribers\".\"skill_id\" 
    INNER JOIN \"positions\" ON \"positions\".\"skill_id\" = \"skills\".\"id\" 
    WHERE (positions.created_at > subscribers.created_at AND positions.created_at > '2011-02-07 10:15:27.098261')"

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Tis weird that you're getting different results locally. Just at a guess, Heroku uses a postgresql database by default even if you specify something else in your database.yml I believe.

It might be that your local database understands your SQL differently to postgresql or is joining tables differently..

What database are you using on your local machine?

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Need to see the associations between Subscriber, Skill, and Position.

Did you really mean to do join with nested associations, or did you mean to do a join of Subscriber with multiple assocations, joins(:skills, :positions).