CODEWITHSUNDEEP
mongodbmongodb-queryaggregation-framework
Locohost
Locohost

Reputation: 1702

MongoDb aggregation $lookup with foreign _ids in arrays

I'm a MongoDb novice. I'm getting pretty good, but no expert yet. I'm trying setup my collections in a way that makes sense. I'd like to keep some links to foreign docs inside arrays of just _ids and also arrays of objects that have _ids.

I created a JSON doc with notes that I think fully shows what I'm trying to do...

// ( item ) Character Inventory/Items collection
[
    {
        "_id": "1234",
        "name": "Sword",
        "descr": "Long sword, well worn, light rust",
        "encumber": 2,
        "del": false
    },
    {
        "_id": "1271",
        "name": "Pouch",
        "descr": "Small leather waist pouch, suitable for coins",
        "encumber": 0,
        "del": false
    }
]

// ( charnpcclass ) Character Classes collection
[
    { "_id": "2", "name": "Thief", "del": false },
    { "_id": "3", "name": "Cleric", "del": false }
]

// ( charnpcalign ) Character Alignments collection
[
    { "_id": "3", "name": "Lawful Good", "del": false },
    { "_id": "4", "name": "Neutral", "del": false }
]

// ( character ) Characters collection
[
    {
        "_id": "3345",
        "name": "Offut 'Dead Dog' Dubro",
        "description": "Halfling, scruffy, looks homeless",
        "align": ObjectId("4"),
        "classes": [
            ObjectId("2"),
            ObjectId("3")
        ],
        "carrying": [
            { "itemId": ObjectId("1271"), "qty":1, "where": "Sheath inside vest", "visible": false }
            { "itemId": ObjectId("1234"), "qty":1, "where": "Sword scabbard at waist", "visible": true }
        ],
        "del": false
    }
]

// ------------------------------------------------------------
// This is my MongoDb aggregation in the REST api routes

var linkedModels = [
    {
        "$match": { "del": false }
    }, {
        "$lookup": {
            from: "charnpcclass",
            localField: "classes",
            foreignField: "_id",
            as: "linked_classes"
        }
    }, {
        "$lookup": {
            from: "charnpcalign",
            localField: "alignId",
            foreignField: "_id",
            as: "linked_align"
        }
    }, {
        "$lookup": {
            from: "item",
            localField: "carrying.itemId",
            foreignField: "_id",
            as: "linked_carrying"
        }
    }
];
db.collection('character').aggregate(linkedModels).toArray(function (err, docs) {
    res.json(201, docs);
    next();
});


// Query for Character, return items carrying with data from items collection

// ------------------------------------------------------------
// WHAT I *WANT* IN RESPONSE...
{
    "id": "3345",
    "name": "Offut 'Dead Dog' Dubro",
    "description": "Halfling, scruffy, looks homeless",
    "align": "4",
    "classes": [
        "2",
        "3"
    ],
    "carrying": [
        { "itemId": "1271", "qty":1, "where": "Sheath inside vest", "visible": false }
        { "itemId": "1234", "qty":1, "where": "Sword scabbard at waist", "visible": true }
    ],
    "linked_align": [
        { "_id": "4", "name": "Neutral" },
    ],
    "linked_classes": [
        { "_id": "2", "name": "Thief" },
        { "_id": "3", "name": "Cleric" }
    ],
    "linked_carrying": [
        { "_id": "1271", "name": "Dagger", "encumber": 0 },
        { "_id": "1234", "name": "Sword", "encumber": 2 }
    ]
}

// ------------------------------------------------------------
// WHAT I ACTUALLY GET IN RESPONSE
{
    "id": "3345",
    "name": "Offut 'Dead Dog' Dubro",
    "description": "Halfling, scruffy, looks homeless",
    "align": "4",
    "classes": [
        "2",
        "3"
    ],
    "carrying": [
        { "itemId": "1271", "qty":1, "where": "Sheath inside vest", "visible": false }
        { "itemId": "1234", "qty":1, "where": "Sword scabbard at waist", "visible": true }
    ],
    "linked_align": [
        { "_id": "4", "name": "Neutral" },
    ],
    "linked_classes": [],
    "linked_carrying": []
}

The problem that I hope you noticed is just above, at bottom of JSON response example. My linked arrays are empty and I'm not sure how to solve this.

I would greatly appreciate your expert MongoDb querying advice :-)

Upvotes: 1

Views: 4587

Answers (1)

s7vr
s7vr

Reputation: 75984

You have to $unwind to flatten the both scalar and sub document foreign _ids and add $group stage at the end of the pipeline to get back the original structure.

$first accumulator to keep the fields and $push with $arrayElemAt to accumulate the array values to adjust for $unwind

var linkedModels = [
    {
        "$match": { "del": false }
    }, 
    {
        "$lookup": {
            from: "charnpcalign",
            localField: "align",
            foreignField: "_id",
            as: "linked_align"
        }
    }, 
    {
        "$unwind":"$classes"
    },
    {
        "$lookup": {
            from: "charnpcclass",
            localField: "classes",
            foreignField: "_id",
            as: "linked_classes"
        }
    },
    {
        "$group": {
           "_id": "$_id",
           "name": {"$first":"$name"},
           "align": {"$first":"$align"},
           "classes":{"$push":"$classes"},
           "carrying":{"$first":"$carrying"},
           "linked_align":{"$first":"$linked_align"},
           "linked_classes":{"$push":{"$arrayElemAt":["$linked_classes",0]}}
        }
    },
    {
        "$unwind":"$carrying"
    },
    {
        "$lookup": {
            from: "item",
            localField: "carrying.itemId",
            foreignField: "_id",
            as: "linked_carrying"
        }
    },
    {
        "$group": {
           "_id": "$_id",
           "name": {"$first":"$name"},
           "align": {"$first":"$align"},
           "classes":{"$first":"$classes"},
           "linked_align":{"$first":"$linked_align"},
           "carrying":{"$push":"$carrying"},
           "linked_carrying":{"$push":{"$arrayElemAt":["$linked_carrying",0]}}
        }
    }
]

You don't need the $unwind on the scalar array (classes) in 3.4 version and you can replace the {"classes":{"$push":"$classes"}} & {"linked_classes":{"$push":{$arrayElemAt:["$linked_classes",0]}}} with {"classes":{"$first":"$classes"}} & {"linked_classes":{"$first":"$linked_classes"}} respectively.

Upvotes: 4

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