mongodb 聚合“$in”运算符：如何寻址数组内所有嵌套数组元素的路径

Question

So i want to check if an element is in few arrays. the number of arrays can change between 2 to 4. the structure of data is as follows:

    {"question" : {
    "answers" : [ 
        {
            "answerText" : "7000",
            "_answerVotes" : []
        }, 
        {
            "answerText" : "15000",
            "_answerVotes" : []
        }, 
        {
            "answerText" : "45000",
            "_answerVotes" : []
        }, 
        {
            "answerText" : "80000",
            "_answerVotes" : []
        }
    ],
    "expireTime" : ISODate("2022-12-14T15:58:11.453Z")
}}

aggregation stage:

        (addFields.isUserVotedAnswerd = {
      $cond: {
        if: {
          $in: [ObjectId(user), "question.answers.$._answerVotes"],
        },
        //   $or: [
        //     { $in: [ObjectId(user), "$question.answers.0._answerVotes"] },
        //     { $in: [ObjectId(user), "$question.answers.1._answerVotes"] },
        //     { $in: [ObjectId(user), "$question.answers.2._answerVotes"] },
        //     { $in: [ObjectId(user), "$question.answers.3._answerVotes"] },
        //   ],
        // },
        then: true,
        else: false,
      },
    }),

Tried both approaches. And I keep getting this error: "$in requires an array as a second argument, found: string "

Anybody know how to address this? Many thanks

Answer 1

Given this input:

var r = [
    {question: {
        "answers" : [
            {"answerText" : "7000", "_answerVotes" : ["U1"] },
            {"answerText" : "15000", "_answerVotes" : ["U2","U7"] }
    ]
    }},

    {question: {
         "answers" : [
             {"answerText" : "7000", "_answerVotes" : ["U2"] },
             {"answerText" : "15000", "_answerVotes" : ["U9"] }
         ]
    }},

    {question: {
         "answers" : [
             {"answerText" : "7000", "_answerVotes" : ["U7"] }
         ]
    }}

];

then the following will select only those docs where at least one user ID in any of the _answerVotes in the answers array matches the target:

var target_user = 'U2';  // user is string here but could be ObjectId

db.foo.aggregate([
    {$project: {'X': {$filter: {input: '$question.answers',
                                as: 'z',
                                cond: {$in:[target_user,'$$z._answerVotes']}
                     }}
    }}
    ,{$match: {$expr: {$ne:[0,{$size:'$X'}] } }}
]);