ActiveRecord left join

active_record_extension.rb

module ActiveRecordExtension
  extend ActiveSupport::Concern

  module ClassMethods
    # Simple left join taking advantage of existing Rails & Arel code
    def left_joins(*args)
      inner_joins = self.joins(*args).arel.join_sources
      left_joins = inner_joins.map do |join|
        Arel::Nodes::OuterJoin.new(join.left, join.right)
      end
      self.joins(left_joins)
    end

    # Find records of self where no records of given association exist
    def without(assoc_name)
      assoc = reflect_on_association(assoc_name)
      left_joins(assoc_name).where(assoc.table_name => {assoc.klass.primary_key => nil})
    end
  end
end

ActiveRecord::Base.send(:include, ActiveRecordExtension)

It may also be sufficient to use

unscope(:joins).arel.join_sources