sql - 将 sql 查询覆盖到 Arel

Question

如何将以下 sql 查询转换为 Arel？

SELECT 
  SUM(
    LEAST(
      SPONSORSHIPS.AMOUNT_PER_LAP * (
        SELECT 
          SUM(LAPS.POINTS) 
        FROM 
          LAPS 
        WHERE 
          LAPS.CAMPAIGN_ID = :j_id
      ),
      SPONSORSHIPS.MAX_AMOUNT
    )
  ) 
FROM 
  SPONSORSHIPS 
WHERE 
  SPONSORSHIPS.CAMPAIGN_ID = :j_id;

有3个表/模型。jogathons, sponsorships, 和laps. :j_id是jogathon id。