How to find and group by a field that is inside a sub-collection?

Question

How to find and group by a field that is inside a sub-collection?

Collection:

[{
    _id: "Jon"
    gender: "male",
    sockets: [
        { code: 12345, type: 'default' },
        { code: 67891, type: 'special' }
    ]
},
{
    _id: "Jane"
    gender: "female",
    sockets: [
        { code: 445566, type: 'very_special' },
        { code: 223388, type: 'extra_special' }
    ]
}]

I want to run a search for the code field. So the user starts typing a code and should be suggested with found codes. E.g. when typing 1 results should be

[{ 
    _id: "Jon", code: 12345
}, {
    _id: "Jon", code: 67891
}]

I know I have to use aggregation with grouping. But I cannot find any example how to do this for my use case. If I would group by a standard field I would use this. But this does not apply to a sub-collection like sockets:

collection.aggregate([{
    $group: {
       _id: "$code"
    }
}]);

Any ideas?

node.js
mongodb
mongodb-query
aggregation-framework

Answer 1

When you deal with "arrays" in the aggregation framework you generally want to $unwind first. This effectively "de-normalizes" your document into separate documents for each array entry.

The other thing here for the type of "grouping" you possibly want is that _id can contain a combination of fields to "group" on rather than just a single field value. Possibly no difference from just processing $unwind here in this example, but for an application of $group you do it like this:

db.collection.aggregate([
    { "$unwind": "$sockets" },
    { "$group": {
        "_id": { "_id": "$_id", "code": "$sockets.code" }
    }}
])

Generally you actually really want to "group" on something, but perhaps you just want the "de-normalized" form, which you can also do with $project:

db.collection.aggregate([
    { "$unwind": "$sockets" },
    { "$project": {
        "_id": 1,
        "code": "$sockets.code"
    }}
])