如何从两个 MongoDb 集合计算 array.reduce

Question

I have two Mongo Db collection. Relation is 1 to 1. I need to make array.reduce on base of both this collection. Something like:

CollectionA: [
  { _id: 1, valueA: 234, idB: 1 },
  { _id: 2, valueA: 64, idB: 2 },
  { _id: 3, valueA: 456, idB: 3 },
]

CollectionB: [
  { _id: 1, valueB: 678 },
  { _id: 2, valueB: 11 },
  { _id: 3, valueB: 4005 },
]

I need to get sum of:

sum(valueA - valueB)

Is it possible to do with one request to Database? If not what is the most elegant way?

Answer 1

You can use aggregation-pipeline to get the results you are looking for.

Your query should look something like this:

db.CollectionA.aggregate([
    {
        $lookup: {
            from: 'B',
            localField: 'idB',
            foreignField: '_id',
            as: 'objB'
        }
    },
    { $unwind: '$objB' },
    {
        $project: {
            subField: {
                $subtract: ['$valueA', '$objB.valueB']
            }
        }
    },
    {
        $group: {
            _id: null,
            sum: {
                $sum: '$subField'
            }
        }
    },
    {
        $project: {
            _id: 0
        }
    }
]);