Question

In a rails application I am working on I have the following setup:

class Volunteer < AR::Base
  has_many :engagements
end

class Engagement < AR::Base
  belongs_to :volunteer
end

I am trying to select all volunteers who are not engaged on a particular date or between date ranges:

scope :not_engaged_on, lambda { |q| joins(:engagements).where(["engagements.date != ?", q])}

scope :not_engaged_between, lambda { |s, e| joins(:engagements).where(["engagements.date NOT BETWEEN ? AND ?", s, e]) }

These scopes I am merging along with other scopes to filter volunteers.

The above scope :not_engaged_between produces the following SQL:

SELECT "volunteers".* FROM "volunteers" INNER JOIN "accounts" ON "accounts"."user_id" = "volunteers"."id" AND "accounts"."user_type" = 'Volunteer' INNER JOIN "engagements" ON "engagements"."volunteer_id" = "volunteers"."id" WHERE (accounts.activation_state = 'active') AND (engagements.date NOT BETWEEN '2012-07-30' AND '2012-08-04')

The problem is a Volunteer might not have any engagements till now and in that case the query totally ignores those volunteers. How can I make return all volunteers even if they don’t have any engagements?

Answer 1

You need a not exists to do this. This syntax is a bit heavy on SQL, maybe there’s a better arel-way to do it.

scope :not_engaged_on, lambda { |q| where(["NOT EXISTS (SELECT ID from engagements WHERE engagements.date = ? AND volunteer_id = volunteers.id)", q])}

scope :not_engaged_between, lambda { |s, e| where(["NOT EXISTS (SELECT ID from engagements WHERE engagements.date BETWEEN ? AND ? AND volunteer_id = volunteers.id)", s, e]) }