.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[{}]