A hack to make `where(relation.exists)` work in 4.2. See https://github.com/rails/rails/issues/16959 for details of the problem and proposed proper solution.

active_record_relation_exitsts.rb

module ActiveRecordRelationExists

  # DANGER this switches ActiveRecord::Relation#exists from delegation to arel to our implementation that returns a String with a twist.
  #  This achieves the most common usage of #exists before 4.2. Proper fix will be: https://github.com/rails/rails/issues/16959
  def exists
    SqlStringWithNot.new("EXISTS (#{self.to_sql})")
  end

  class SqlStringWithNot < String

    def not
      self.class.new("NOT (#{self})")
    end

    def to_sql
      self.to_s
    end

  end

end
ActiveRecord::Relation.send(:include, ActiveRecordRelationExists)