need AR query for null parent field or non-null parent field and no related child records

chores_controller.rb

def search
  @chores = Chore.available
  if search_params[:search_text]
    @chores = @chores.where('description LIKE ?', "%%%s%" % search_params[:search_text])
  elsif search_params[:zipcode]
    @chores = @chores.where('location LIKE ?', "%%%s%" % search_params[:zipcode])
  end

  @chores = @chores.order(created_at: :desc).all

  render 'index'
end

log.txt

Chore Load (0.2ms)  SELECT "chores".* FROM "chores" INNER JOIN "taken_chores" ON "taken_chores"."chore_id" = "chores"."id" WHERE (NOT (deadline is not null)) AND (description LIKE '%keyword%') ORDER BY "chores"."created_at" DESC

parent.rb

class Parent < ActiveRecord::Base
  has_many :children
  scope :available, -> { joins(:taken_chore).where.not('deadline is not null', :taken_chores => { :chore_id => !nil })}
end

query.sql

select * from chores
where chores.deadline is null 
OR (chores.deadline is not null AND 
   (select count(*) from taken_chores where taken_chores.chore_id=chores.id) = 0)
order by chores.created_at desc