我通过查询从嵌套数组文档中检索唯一的对象数组，所以我在其中使用了聚合和$group。
我的数据结构

{
   _id: ObjectId('637b22639356492ae41bbe23'),
   studentAgeCategories: [
       {
          competitionsType: [
           {
                    competitionsTypeId: ObjectId("5fec0c53a534c297c6b4c5c3"),
                    competitionsTypeName: "Animation/Documentary/Film"
            }
          ]
       }
   ]
}

尝试的查询

{'$match': {'_id': ObjectId('637b22639356492ae41bbe23')}},
{'$unwind': '$studentAgeCategories'},
{'$group': {
    '_id': '$studentAgeCategories.competitionsType.competitionsTypeId',
    'competitionType': {
        '$push': {
            '_id': '$studentAgeCategories.competitionsType.competitionsTypeId',
            'competitionsTypeName': '$studentAgeCategories.competitionsType.competitionsTypeName'
            }
        }
    }
}

dbconn.clnEvents.aggregate(pipeline)

这就是结果。

/* 1 */
{
    "_id" : [
        ObjectId("5fec0c53a534c297c6b4c5c3"),
        ObjectId("5fec0c65a534c297c6b4ce3a"),
        ObjectId("5fec0c8aa534c297c6b4dda7")
    ],
    "competitionType" : [
        {
            "_id" : [
                ObjectId("5fec0c53a534c297c6b4c5c3"),
                ObjectId("5fec0c65a534c297c6b4ce3a"),
                ObjectId("5fec0c8aa534c297c6b4dda7")
            ],
            "competitionsTypeName" : [ "Animation/Documentary/Film", "Visual coding", "Websites/Web/Mobile Apps" ]
        }
    ]
},

/* 2 */
{
    "_id" : [
        ObjectId("5ff457b2add6eab7491fff13")
    ],
    "competitionType" : [
        {
            "_id" : [
                ObjectId("5ff457b2add6eab7491fff13")
            ],
            "competitionsTypeName" : [ "Presentation" ]
        }
    ]
},

/* 3 */
{
    "_id" : [
        ObjectId("5fec0c9fa534c297c6b4e4b6")
    ],
    "competitionType" : [
        {
            "_id" : [
                ObjectId("5fec0c9fa534c297c6b4e4b6")
            ],
            "competitionsTypeName" : [ "AI/Robotics/IoT" ]
        }
    ]
},

/* 4 */
{
    "_id" : [
        ObjectId("60d8843f6ac43b6179d3bd7e"),
        ObjectId("5ff457b2add6eab7491fff13"),
        ObjectId("5fec0c53a534c297c6b4c5c3")
    ],
    "competitionType" : [
        {
            "_id" : [
                ObjectId("60d8843f6ac43b6179d3bd7e"),
                ObjectId("5ff457b2add6eab7491fff13"),
                ObjectId("5fec0c53a534c297c6b4c5c3")
            ],
            "competitionsTypeName" : [ "Programming Competition", "Presentation", "Animation/Documentary/Film" ]
        }
    ]
},

/* 5 */
{
    "_id" : [
        ObjectId("60d8843f6ac43b6179d3bd7e")
    ],
    "competitionType" : [
        {
            "_id" : [
                ObjectId("60d8843f6ac43b6179d3bd7e")
            ],
            "competitionsTypeName" : [ "Programming Competition" ]
        }
    ]
}

相反，我需要得到这样的东西。

"competitionType" : [
    
{"_id" :ObjectId("5fec0c9fa534c297c6b4e4b6"),
 "competitionsTypeName" : "AI/Robotics/IoT" },

{"_id" :ObjectId("5fec0c9fa534c297c6b4e4b6"),
 "competitionsTypeName" : "AI/Robotics/IoT" },

{"_id" :ObjectId("5fec0c9fa534c297c6b4e4b6"),
 "competitionsTypeName" : "AI/Robotics/IoT" }]  
]

在浏览了一些mongodb的文章和论坛后，我迷路了，现在我不知道我错过了什么。