Project only some fields of array items in sub document - mongodb

Question

How can I project only particular fields of items in array in sub document?
Consider the following (simplified) example:
{
"_id" : ObjectId("573d70df080cc2cbe8bf3222"),
"name" : "Nissan",
"models" : [
{
"name" : "Altima",
"body" : {
"type" : 2,
"maxprice" : 31800.00,
"minprice" : 21500.00
}
},
{
"name" : "Maxima",
"body" : {
"type" : 2,
"maxprice" : 39200.00,
"minprice" : 28800.00
}
}
]
},
{
"_id" : ObjectId("80cc2cbe8bf3222573d70df0"),
"name" : "Honda",
"models" : [
{
"name" : "Accord",
"body" : {
"type" : 2,
"maxprice" : 34100.00,
"minprice" : 20400.00
}
},
{
"name" : "Civic",
"body" : {
"type" : 3,
"maxprice" : 27900.00,
"minprice" : 19800.00
}
}
]
}
After aggregation, I'd like to get the following output:
{
"_id" : ObjectId("573d70df080cc2cbe8bf3222"),
"name" : "Nissan",
"models" : [
{
"type" : 2,
"minprice" : 21500.00
},
{
"type" : 2,
"minprice" : 28800.00
}
]
},
{
"_id" : ObjectId("80cc2cbe8bf3222573d70df0"),
"name" : "Honda",
"models" : [
{
"type" : 2,
"minprice" : 20400.00
},
{
"type" : 3,
"minprice" : 19800.00
}
]
}
So it basically gets all documents, all fields of documents, all items in models array, BUT only some fields of the array items in models. Please help.

Answer 1

You need to $project the "models" field using the $map operator.
db.collection.aggregate([
{ "$project": {
"name": 1,
"models": {
"$map": {
"input": "$models",
"as": "m",
"in": {
"type": "$$m.body.type",
"minprice": "$$m.body.minprice"
}
}
}
}}
])

Answer 2

$unwind is your friend
First you can basically filter the (non nested) fields you want.
var projection = {$project:{name:'$name', models:'$models'}};
db.dum.aggregate(projection)
Foreach of your models, you issue a document
var unwindModels = {$unwind:{'$models'}}
db.dum.aggregate(projection, unwindModels)
The idea is that every document issued from your models field will be regrouped later on via the _id field.
Foreach document, you only keep the (sub)fields you want
var keepSubFields = {$project:{name:'$name', type:'$models.body.type', minprice:'$models.body.minprice'}}
db.dum.aggregate(projection, unwindModels, keepSubFields)
Then you reaggregate your models as an array (thanks to the _id of each record which tracks the original record)
var aggregateModels = {$group:{_id:'$_id', name:{$last:'$name'}, models:{$push:{type:'$type', minprice:'$minprice'}}}}
db.dum.aggregate(projection, unwindModels, keepSubFields, aggregateModels)
note1: Here we can use $last because our primary key is not _id but <_id, name>. ($first would be good too)
note2: we refer type by $type, because when you iterate the collection on the aggregateModels stage, your record is of the form
<_id, name, type, minprice>