MongoDb-aggregatie $lookup met buitenlandse _ids in arrays

Je moet $unwind om de buitenlandse _ids van zowel scalair als subdocument af te vlakken en $group . toe te voegen fase aan het einde van de pijplijn om de oorspronkelijke structuur terug te krijgen.

$first accumulator om de velden en $push . te behouden met $arrayElemAt om de arraywaarden te verzamelen om aan te passen voor $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]}}
        }
    }
]

Je hebt de $unwind . niet nodig op de scalaire array (classes ) in versie 3.4 en u kunt de {"classes":{"$push":"$classes"}} vervangen &{"linked_classes":{"$push":{$arrayElemAt:["$linked_classes",0]}}} met {"classes":{"$first":"$classes"}} &{"linked_classes":{"$first":"$linked_classes"}} respectievelijk.