Question

I’m having trouble figuring out the scope method for all the Foos that have no Bars. That is:

class Foo < ActiveRecord::Base

  has_may :bars

end

class Bar < ActiveRecord::Base

  belongs_to :foo

end

I’d like to write a scope method that returns me all the foos that have no bars. Something like:

class Foo < ActiveRecord::Base

  has_may :bars

  scope :has_no_bars, includes(:bars).where("COUNT(foo.bars) = 0")

end

But I don’t understand the appropriate syntax. Any help? Happy to use a MetaWhere solution if easier.

Answer 1

You need either a sub-select or an outer join + group + unique to solve your problem. AFAIK that is not possible with Rails’ AR.

An approximation would be to use AR’s counter-cache feature and make your query as simple as

scope :has_no_bars, where("bars_count = 0")

This isn’t 100% correct relation-wise but saves you a lot of work and also scales much better.