MongoDB / Mongo aggregate example - mapReduce, aggregate query

mongo-aggregate.js

db.getCollection('routes').aggregate([
    {
        $match: {city: 'Pune', transport: 'subway'}
    },
    {
        $project: {emit: {key: {city: '$city', transport: '$transport'}, features: '$featureCollection.features'}}
    },
    {
        $group: {
            _id: "$emit.key", 
            routes: {
                $accumulator: {
                    init: function() { return []; },
                    accumulate: function(state, value) { return Object.assign([], state, value); },
                    accumulateArgs: [ "$emit.features" ],
                    merge: function(state1, state2) { return Object.assign([], state1, state2); },
                    finalize: function(state) { 
                        return state.filter(function(item){ return item.properties.type === 'relation' })
                            .map(function(item){
                                return {
                                  id: item.properties.id,
                                  name: item.properties.tags.name,
                                  from: item.properties.tags.from,
                                  to: item.properties.tags.to,
                                  network: item.properties.tags.network,
                                  operator: item.properties.tags.operator,
                                  geometry: item.geometry,
                                  stops: state.filter(function(stop){
                                    return stop.properties.type === 'node' && stop.properties.relations.find(function(relation){
                                        return relation.rel === item.properties.id;
                                    })
                                  }).map(function(stop){
                                        return {
                                            id: stop.properties.id,
                                            name: stop.properties.tags.name,
                                            location: stop.geometry,
                                        }
                                    })
                                }
                            }) 
                    }
                }
            }
        }
    }
])