Mongodb$graphLookupビルド階層

以下の解決策は、私の過去の回答の1つとほぼ同じであるため、ここで

db.projects.aggregate([
    {
        $graphLookup: {
            from: "projects",
            startWith: "$_id",
            connectFromField: "_id",
            connectToField: "parent",
            as: "children",
            maxDepth: 4,
            depthField: "level"
        }
    },
    {
        $unwind: "$children"
    },
    {
        $sort: { "children.level": -1 }
    },
    {
        $group: {
            _id: "$_id",
            children: { $push: "$children" }
        }
    },
    {
        $addFields: {
            children: {
                $reduce: {
                    input: "$children",
                    initialValue: {
                        currentLevel: -1,
                        currentLevelProjects: [],
                        previousLevelProjects: []
                    },
                    in: {
                        $let: {
                            vars: {
                                prev: { 
                                    $cond: [ 
                                        { $eq: [ "$$value.currentLevel", "$$this.level" ] }, 
                                        "$$value.previousLevelProjects", 
                                        "$$value.currentLevelProjects" 
                                    ] 
                                },
                                current: { 
                                    $cond: [ 
                                        { $eq: [ "$$value.currentLevel", "$$this.level" ] }, 
                                        "$$value.currentLevelProjects", 
                                        [] 
                                    ] 
                                }
                            },
                            in: {
                                currentLevel: "$$this.level",
                                previousLevelProjects: "$$prev",
                                currentLevelProjects: {
                                    $concatArrays: [
                                        "$$current", 
                                        [
                                            { $mergeObjects: [ 
                                                "$$this", 
                                                { children: { $filter: { input: "$$prev", as: "e", cond: { $eq: [ "$$e.parent", "$$this._id"  ] } } } } 
                                            ] }
                                        ]
                                    ]
                                }
                            }
                        }
                    }
                }
            }
        }
    },
    {
        $addFields: { children: "$children.currentLevelProjects" }
    },
    {
        $match: {
            _id: "1"
        }
    }
])

最後の段階はフィルタリングであると想定されているため、ここで任意のレベルの深度のデータを取得できます。