Project only some fields of array items in sub document

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" 
                } 
            } 
        }  
    }} 
])