MongoDB flatten embedded array - mongodb

Question

i'd like to create a report of a collection. Its schema is :
(I simplified the schema, to focus on the problematic)
Mongoose Schema
var MobilHomeSchema = new Schema({
id: Schema.Types.ObjectId,
region: String,
equipments:[
{ id: ObjectId, label: String }
]
});
It contains lots of mobilhomes. These mobilhomes are in a campsite, on a region (I chose this group, it could be country, ...). Each mobilhome has some equipments, not always the sames.
I'd like to create a spreadsheet with these columns, to count the number of each equipments in a region (it's just an example)
Expected generic result format
region | equipments.label 1 | equipments.label 2 | equipments.label 3 | ....
Example with "real" values :
region|terrace|pergola|shower
Spain | 30 | 15 |150
France| 55 | 32 |540
...
in json format, it could be :
EDIT
[{
region: "Spain",
terrace: 30,
pergola: 15,
shower: 150
},
{
region: "France",
terrace: 55,
pergola: 32,
shower: 540
}]
/EDIT
How can I do ?
(map-reduce ? a most Business Intelligence tool ?)
Many Thanks !

Answer 1

Don't use map/reduce. Use aggregation. In the mongo shell,
> db.mobile.aggregate([
{ "$unwind" : "$equipments" },
{ "$group" : { "_id" : { "region" : "$region", "label" : "$equipments.label" }, "count" : { "$sum" : 1 } } }
])
On the documents
{ "region" : "France", "equipments" : [ { "_id" : 0, "label" : "terrace" }, { "_id" : 1, "label" : "pergola" } ] },
{ "region" : "France", "equipments" : [ { "_id" : 0, "label" : "shower" }, { "_id" : 1, "label" : "pergola" } ] },
{ "region" : "Spain", "equipments" : [ { "_id" : 0, "label" : "terrace" }, { "_id" : 1, "label" : "shower" } ] },
{ "region" : "Spain", "equipments" : [ { "_id" : 0, "label" : "veranda" }, { "_id" : 1, "label" : "pergola" } ] }
the result is
{ "_id" : { "region" : "Spain", "label" : "veranda" }, "count" : 1 }
{ "_id" : { "region" : "Spain", "label" : "terrace" }, "count" : 1 }
{ "_id" : { "region" : "Spain", "label" : "shower" }, "count" : 1 }
{ "_id" : { "region" : "France", "label" : "shower" }, "count" : 1 }
{ "_id" : { "region" : "France", "label" : "pergola" }, "count" : 2 }
{ "_id" : { "region" : "Spain", "label" : "pergola" }, "count" : 1 }
{ "_id" : { "region" : "France", "label" : "terrace" }, "count" : 1 }
Since you're using an array, presumably you don't know all the possible types of equipment ahead of time, which makes shoving the above results back into one object per region in the aggregation an unwieldy thing to attempt. Better to work with these results in the client.