Question

I am using Rails 3.0. I have two tables: Listings and Offers. A Listing has-many Offers. An offer can have accepted be true or false.

I want to select every Listing that does not have an Offer with accepted being true. I tried

Listing.joins(:offers).where('offers.accepted' => false)

However, since a Listing can have many Offers, this selects every listing that has non-accepted Offers, even if there is an accepted Offer for that Listing.

In case that isn’t clear, what I want is the complement of the set:

Listing.joins(:offers).where('offers.accepted' => true)

My current temporary solution is to grab all of them and then do a filter on the array, like so:

class Listing < ActiveRecord::Base
...
    def self.open
        Listing.all.find_all {|l| l.open? }
    end

    def open?
        !offers.exists?(:accepted => true)
    end
end

I would prefer if the solution ran the filtering on the database side.

Answer 1

The first thing that comes to mind is to do essentially the same thing you’re doing now, but in the database.

scope :accepted, lambda {
  joins(:offers).where('offers.accepted' => true)
} 

scope :open, lambda {
  # take your accepted scope, but just use it to get at the "accepted" ids
  relation = accepted.select("listings.id")

  # then use select values to get at those initial ids
  ids = connection.select_values(relation.to_sql)

  # exclude the "accepted" records, or return an unchanged scope if there are none
  ids.empty? ? scoped : where(arel_table[:id].not_in(ids))
}

I’m sure this could be done more cleanly using an outer join and grouping, but it’s not coming to me immediately 🙂