.lookup({
    from: 'products',
    localField: 'productId',
    foreignField: '_id',
    as: 'product',
  })
  .match({
    created_at:_.gte(new Date(today+" 00:00:00")),
    created_at:_.lte(new Date(today+" 23:59:59"))
  })
  .end()
//期望product 是一个{}而非[{}]