MongoDB Aggregation: Combine two arrays
An alternative solution with neither aggregation framework nor map/reduce:
db.sens.find().forEach(function (doc) { doc.data.forEach(function(dataElement) { var sumArray = []; for (var j = 0; j < dataElement.flow.length; j++) { sumArray[j] = dataElement.flow[j] + dataElement.occupancy[j]; } dataElement.sum = sumArray; collection.save(doc); });});