Aggregation with meteorhacks:aggregate (why would I ever use $out)?

问题

This is a significant edit to this question, as I have changed the publication to a method and narrowed the scope of the question.

I am using meteorhacks:aggregate to calculate and publish the average and median of company valuation data for a user-selected series of companies. The selections are saved in a Valuations collection for reference and the data for aggregation comes from the Companies collection.

The code below works fine for one-time use (although it's not reactive). However, users will rerun this aggregation for thousands of valuationIds. Since $out will first clear out the new collection before inserting the new results, I can't use that here, I need to retain the results of each instance. I don't understand why $out would ever be used.

Is there any way to just add update the existing Valuation document with the aggregation results and then subscribe to that document?

server/methods

Meteor.methods({
    valuationAggregate: function(valuationId, valuationSelections) {
        //Aggregate and publish average of company valuation data for a user-selected series of companies./
        //Selections are saved in Valuations collection for reference and data for aggregation comes from Companies collection.//
        check(valuationId, String);
        check(valuationSelections, Array);
        var pipelineSelections = [
            //Match documents in Companies collection where the 'ticker' value was selected by the user.//
            {$match: {ticker: {$in: valuationSelections}}},
            {
                $group: {
                    _id: null,
                    avgEvRevenueLtm: {$avg: {$divide: ["$capTable.enterpriseValue", "$financial.ltm.revenue"]}},
                    avgEvRevenueFy1: {$avg: {$divide: ["$capTable.enterpriseValue", "$financial.fy1.revenue"]}},
                    avgEvRevenueFy2: {$avg: {$divide: ["$capTable.enterpriseValue", "$financial.fy2.revenue"]}},
                    avgEvEbitdaLtm: {$avg: {$divide: ["$capTable.enterpriseValue", "$financial.ltm.ebitda"]}},
                    //more//
                }
            },
            {
                $project: {
                    _id: 0,
                    valuationId: {$literal: valuationId},
                    avgEvRevenueLtm: 1,
                    avgEvRevenueFy1: 1,
                    avgEvRevenueFy2: 1,
                    avgEvEbitdaLtm: 1,
                    //more//
                }
            }
        ];

        var results = Companies.aggregate(pipelineSelections);
        console.log(results);
        }
});

The code above works, as far as viewing the results on the server. In my terminal, I see:

I20150926-23:50:27.766(-4)? [ { avgEvRevenueLtm: 3.988137239679733,
I20150926-23:50:27.767(-4)?     avgEvRevenueFy1: 3.8159564713187155,
I20150926-23:50:27.768(-4)?     avgEvRevenueFy2: 3.50111769838031,
I20150926-23:50:27.768(-4)?     avgEvEbitdaLtm: 11.176476895728268,
//more//
I20150926-23:50:27.772(-4)?     valuationId: 'Qg4EwpfJ5uPXyxe62' } ]

回答1:

I was able to resolve this with the following. Needed to add the forEach to unwind the array in the same way as $out.

lib/collections

ValuationResults = new Mongo.Collection('valuationResults');

server/methods

    var results = Companies.aggregate(pipelineSelections);
    results.forEach(function(valuationResults) {
        ValuationResults.update({'result.valuationId': valuationId}, {result:valuationResults}, {upsert: true});
    });
    console.log(ValuationResults.find({'result.valuationId': valuationId}).fetch());
}
});

来源：https://stackoverflow.com/questions/32708131/aggregation-with-meteorhacksaggregate-why-would-i-ever-use-out