Solución para encontrar registros duplicados que involucren ITS y relación padre-hijo

El siguiente SQL parece hacer el truco

big_query = "
  SELECT EXISTS (
    SELECT 1
    FROM buyables b1
      JOIN buyables b2
        ON b1.shop_week_id = b2.shop_week_id
        AND b1.location_id = b2.location_id
    WHERE
      b1.parent_id != %1$d
      AND b2.parent_id = %1$d
      AND b1.type = 'Item'
      AND b2.type = 'Item'
    GROUP BY b1.parent_id
    HAVING COUNT(*) = ( SELECT COUNT(*) FROM buyables WHERE parent_id = %1$d AND type = 'Item' )
  )
"

Con ActiveRecord, puede obtener este resultado usando select_value :

class Basket < Buyable
  def has_duplicate
    !!connection.select_value( big_query % id )
  end
end

Sin embargo, no estoy tan seguro sobre el rendimiento