Mongoose: Populating object's nested array with projection

Question

var CarSchema = new Schema({
    name: {type: String},
    partIds: [{type: Schema.Types.ObjectId, ref: 'Part'}],
});

var PartSchema = new Schema({
    name: {type: String},
    props: [
        { colour: {type: String}, shape: {type:String} }
    ],
});

For example

Car = {
       name: "BMW",
       partIds:[ObjectId("57baa43e152654f80aac36a6")]}

Part = {
        _id: ObjectId("57baa43e152654f80aac36a6"),
        name: "Piston",
        props: [{colour:"red", shape: "Cubical"},{colour:"green", shape: "cylindrical"}]

So when I query I should get a doc like this:

Car = {
      name: "BMW",
      partIds: [{ 
_id:ObjectId("57baa43e152654f80aac36a6"), name:"Piston", props: [{colour:"red", shape:"cubical"}]
     }

The props array should only have element with colour red

I want to populate Car with Part Array such that its prop array only have the object with colour red. Is there anyway to do it, or would I have to go old fashioned way and loop through props array matching its colour with red.

Do you already have the part Collection? Or do you create it and store its id in car Collection? — Ravi Shankar Bharti
– Ravi Shankar Bharti, Commented Aug 25, 2016 at 7:47
Part collection is different and its Ids are stored in Car collections' partIds — Dark Lord
– Dark Lord, Commented Aug 25, 2016 at 7:49
so you want to create part document and store its id in car collection if the color is red? — Ravi Shankar Bharti
– Ravi Shankar Bharti, Commented Aug 25, 2016 at 8:32
No, I want to query the Car collection such that it populate partIds such that props aray only have elements whose colour is red. — Dark Lord
– Dark Lord, Commented Aug 25, 2016 at 8:44

JohnnyHK · Accepted Answer · 2016-08-27 13:45:55Z

1

You can do this by providing the select option in your populate call:

Car.findOne()
    .populate({
        path: 'partIds',
        select: { props: { $elemMatch: { colour: 'red' } }, name: 1 }
    })
    .exec(callback);

Result:

{ _id: 57c085451cd8dfcdf814f640,
    name: 'BMW',
    partIds:
     [ { _id: 57baa43e152654f80aac36a6,
         name: 'Piston',
         props: [ { colour: 'red', shape: 'Cubical' } ] } ] }

The select uses the $elemMatch projection operator to select just the red props element.

edited Aug 27, 2016 at 13:45

answered Aug 26, 2016 at 18:20

JohnnyHK

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Comments

Ravi Shankar Bharti · Accepted Answer · 2016-08-25 11:22:17Z

0

you can try this, it should get you what you want.

car.find({"partIds.props.colour" : "red"})
    .populate('partIds')
    .exec(function(err,result){...});

answered Aug 25, 2016 at 11:22

Ravi Shankar Bharti

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