如何在mongodb中查找嵌套文档

Question

我有两个文档appointment和assistant。

约会

{
    "_id" : ObjectId("5a4e5a448b70d50e34d204a4"),
    "name" : "Dreikoenigstunier",
    "date" : ISODate("2013-01-06T00:00:00.000Z"),
    "comment" : null,
    "tasks" : [ 
        {
            "name" : "Speisen und Getränke",
            "sections" : [ 
                {
                    "start" : 37800,
                    "end" : 55800,
                    "entirely" : true,
                    "assistants" : [ 
                        {
                            "assistant" : ObjectId("5a4e5a438b70d50e34d203e3"),
                            "state" : 3
                        }, 
                        {
                            "assistant" : ObjectId("5a4e5a438b70d50e34d203e4"),
                            "state" : 3
                        }, 
                        {
                            "assistant" : ObjectId("5a4e5a438b70d50e34d203e5"),
                            "state" : 3
                        }
                    ]
                }, 
                {
                    "start" : 55800,
                    "end" : 68400,
                    "entirely" : true,
                    "assistants" : [ 
                        {
                            "assistant" : {
                                "_id" : ObjectId("5a4e5a438b70d50e34d203d5")
                            },
                            "state" : 3
                        }, 
                        {
                            "assistant" : {
                                "_id" : ObjectId("5a4e5a438b70d50e34d20408")
                            },
                            "state" : 3
                        }, 
                        {
                            "assistant" : {
                                "_id" : ObjectId("5a4e5a438b70d50e34d2040a")
                            },
                            "state" : 3
                        }
                    ]
                }
            ]
        }, 
        {
            "name" : "Schiri",
            "sections" : [ 
                {
                    "start" : 55800,
                    "end" : 68400,
                    "entirely" : true,
                    "assistants" : [ 
                        {
                            "assistant" : {
                                "_id" : ObjectId("5a4e5a438b70d50e34d203d8")
                            },
                            "state" : 3
                        }, 
                        {
                            "assistant" : {
                                "_id" : ObjectId("5a4e5a438b70d50e34d203e6")
                            },
                            "state" : 3
                        }
                    ]
                }
            ]
        }
    ]
}

助手

{
    "_id" : ObjectId("5a4e5a438b70d50e34d203ca"),
    "firstname" : "Max",
    "lastname" : "Mustermann",
    "category" : "Herren 2",
    "email" : "test@test.de",
    "phone" : "0622-...",
    "mobile" : "0151-...",
    "gender" : "male",
    "enabled" : "1",
    "hash" : "$2a$10...",
    "token" : null
}

现在，我需要查询所有约会并查找嵌套的助手属性。 因此，我有以下管道

[{
    $match: {
        "_id": ObjectId("5a4e5a448b70d50e34d204a4")
    }
}, { 
    $unwind: { path: '$tasks', preserveNullAndEmptyArrays: true }
}, { 
    $unwind: { path: '$tasks.sections', preserveNullAndEmptyArrays: true }
}, { 
    $unwind: { path: '$tasks.sections.assistants', preserveNullAndEmptyArrays: true }
}, {
    $lookup: {
        from: 'assistant',
        localField: "tasks.sections.assistants.assistant",
        foreignField: "_id",
        as: "tasks.sections.assistants.assistant"
    }
}, { 
    $unwind: { path: '$tasks.sections.assistants.assistant', preserveNullAndEmptyArrays: true }
}]

但是如何恢复最初的Appointment结构？我知道我必须使用$group，但是如何使用？没有可以分组的标识符...