MongoDB嵌套聚合未找到内部集合

Question

我是第一次接触Stack和MongoDB。我已经根据其他帖子调整了代码，但找不到哪里出错了。我有三个收藏。微生物与活动(1对M)和活动与生物活性(1对M)相连。我想以数组的形式展示来自Microbe的巢直到Bio-Activities。

Microbe collection
{"_id": 1, item: "abc", species: 12, quantity: 2},
{"_id": 2, item: "jkl", species: 20, quantity: 1},
{"_id": 3}

Activities collection
{"_id": 1, code: "MON1003", item: "abc", type: "Monitor", repeat: 120, size: "27", resolution: "1920x1080"},
{"_id": 2, code: "MON1012", item: "abc", type: "Monitor", repeat: 85, size: "23", resolution: "1280x800"},
{"_id": 3, code: "MON1031", item: "jkl", type: "Monitor", repeat: 60, size: "21", resolution: "1920x1080"}


Bio-Activities collection
{"_id": 1, code: "MON1003", description: "bio 1", quantity: 120},
{"_id": 2, code: "MON1003", description: "bio 2", quantity: 80},
{"_id": 3, code: "MON1012", description: "bio 3", quantity: 60},
{"_id": 4, code: "MON1012", description: "bio 4", quantity: 70},
{"_id": 5, code: "MON1012", description: "bio 5", quantity: 170},
{"_id": 6, code: "MON1031", description: "bio 6", quantity: 270}

这是我的代码：

db.microbe.aggregate([ 
{ 
$lookup: 
{ 
from: "activities", 
localField: "item", 
foreignField: "item", 
as: "activities" 
} 
},{
$lookup: 
{ 
    from: "bioactivities", 
    localField: "code", 
    foreignField: "code", 
    as: "bioactivities" 
}
},{
    $unwind: "$activities"
},{
$group: {
    _id: "$_id",
    item: {$first: "$item"},
    species: {$first: "$species"},
    quantity: {$first: "$quantity"},
    activities: {$push: {
        id: "$activities._id",
        code: "$activities.code",
        item: "$activities.item",
        type: "$activities.type",
        repeat: "$activities.repeat",               
        size: "$activities.size",
        bioactivities: "$bioactivities"
    }
    }
}
}
]).pretty()

但是我得到了这个输出，它显示了空的“生物活动”。我哪里错了？

    {
    "_id" : 2,
    "item" : "jkl",
    "species" : 20,
            "quantity" : 1,
            "activities" : [
                    {
                    "id" : 3,
                    "code" : "MON1031",
                    "item" : "jkl",
                    "type" : "Monitor",
                    "repeat" : 60,
                    "size" : "21",
                    "bioactivities" : [ ]
            }
    ]
    }
    {
            "_id" : 1,
            "item" : "abc",
            "species" : 12,
            "quantity" : 2,
            "activities" : [
            {
                    "id" : 1,
                    "code" : "MON1003",
                    "item" : "abc",
                    "type" : "Monitor",
                    "repeat" : 120,
                    "size" : "27",
                    "bioactivities" : [ ]
            },
            {
                    "id" : 2,
                    "code" : "MON1012",
                    "item" : "abc",
                    "type" : "Monitor",
                    "repeat" : 85,
                    "size" : "23",
                    "bioactivities" : [ ]
            }
    ]
    }

谢谢。