How to convert this map reduce in aggregate framework? - mongodb

Question

i've still done this map/reduce/finalize function using mongoDB.
This is how i need that mongoDB executes that aggregation:
db.house_results.mapReduce(function(){
emit(this.house_name.toLowerCase(),this);
},function(key,values){
var house = {name:key,address:"",description:"",photo:[],lat:0,lng:0,rooms:[]};
values.forEach(function(house_val) {
/*Address*/
if(house.address=="")
house.address = house_val.house_address;
/*Photo*/
if(!house_val.photo in house.photo)
house.photo.push(house_val.house_photo);
/*Description*/
if(house.description=="")
house.description = house_val.house_description;
/*LAT - LNG*/
if(house.lat==0 || house.lng==0){
var house_position = house_val.house_position;
if(house_position && house_position.lat && house_position.lng){
house.lat = house_position.lat;
house.lng = house_position.lng;
}
}
if(house.lat==0 || house.lng==0){
if(house_val.house_lat && house_val.house_lng){
house.lat = house_val.house_lat;
house.lng = house_val.house_lng;
}
}
if(house_val.rooms)
house.rooms.push(house_val.rooms);
});
return house;
},
{
out : "map_reduce_house_test",
finalize:function(key,house_val){
if(house_val.address==undefined){ // JUST ONE RESULT IN MAP FUNCTION -> REDUCE FUNCTION IS IGNORED -> FINALIZE IS SOLUTION
var house = {name:key,address:"",description:"",photo:[],lat:0,lng:0,rooms:[]};
/*Address*/
if(house.address=="")
house.address = house_val.house_address;
/*Photo*/
if(!house_val.photo in house.photo)
house.photo.push(house_val.house_photo);
/*Description*/
if(house.description=="")
house.description = house_val.house_description;
/*LAT - LNG*/
if(house.lat==0 || house.lng==0){
var house_position = house_val.house_position;
if(house_position && house_position.lat && house_position.lng){
house.lat = house_position.lat;
house.lng = house_position.lng;
}
}
if(house.lat==0 || house.lng==0){
if(house_val.house_lat && house_val.house_lng){
house.lat = house_val.house_lat;
house.lng = house_val.house_lng;
}
}
if(house_val.rooms)
house.rooms.push(house_val.rooms);
return house;
}else
return house_val;
}
}
);
Is there a way to simplify that functions and/or is better to do the same with aggregation mongodb's function?
Which could be the fastest and simplier method?
Thanks!

Answer 1

There isn't really much going on in this mapReduce other than taking the first values from various fields for the common grouping key and otherwise pushing some other values onto arrays.
Therefore everything is very much the same for aggregation:
db.house_results.aggregate([
{ "$group": {
"_id": { "$toLower": "$house_name" },
"name": { "$first": { "$toLower": "$house_name" } },
"photo": { "$push": "$house_photo" },
"address": { "$first": "$house_address" },
"description": { "$first": "$house_description" },
"lat": {
"$max": {
"$cond": [
{ "$gt": [ "$house_lat", "$house_position.lat" } },
"$house_lat",
"$house_position.lat"
}
},
"lng": {
"$max": {
"$cond": [
{ "$gt": [ "$house_lng", "$house_position.lng" } },
"$house_lng",
"$house_position.lng"
}
},
"rooms": { "$push": "$house_rooms" }
}}
])
The only real difference there is the conditional handling of the "lat" and "lng" output using primarily the $cond operator.
Noting that "_id" and "name" have the same thing in them, but that is what the map reduce is doing.
Take a good look at the aggregation operators for reference, but really your data should look like this rather than it's present form, which appears to be a de-normalized dump from somewhere.
Also for reference, It probably isn't affecting you in this case, but this is the wrong way to write a mapReduce. The output from the "map" function is different to that from the "reduce" function, notably the arrays.
Even though these will only have one element in them they "should" be emitted as an array element from the "map" function as well and treated as if they where already an array element by the "reduce" function.
This is because with larger "grouping", not all matching key values are sent into the reduce function at once, and the reducer can be called to combine other values emitted by "map" to the "reduce" function with previously reduced output. That is how large data is handling, and with arrays you run the risk of output like this, with the un-expected embedding of an array within an array:
[ [4,5,6], 7, 8, 9 ]
But this is covered in the documentation where you read carefully.
At any rate, the aggregation pipeline ( one stage) will perform much faster than the present operation. But really change your data as soon as possible.