Agregacja MongoDb $lookup z obcymi _ids w tablicach

Musisz $unwind spłaszczyć zarówno skalarne, jak i podrzędne obce _ids i dodać $group etap na końcu potoku, aby odzyskać oryginalną strukturę.

$first akumulator do przechowywania pól i $push z $arrayElemAt aby zgromadzić wartości tablicy w celu dostosowania dla $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]}}
        }
    }
]

Nie potrzebujesz $unwind na tablicy skalarnej (classes ) w wersji 3.4 i możesz zastąpić {"classes":{"$push":"$classes"}} &{"linked_classes":{"$push":{$arrayElemAt:["$linked_classes",0]}}} z {"classes":{"$first":"$classes"}} &{"linked_classes":{"$first":"$linked_classes"}} odpowiednio.