Question 196320 in Mongodb Question posted in
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Mongodb – mongoose find query based on priority

I have a studentsubject schema.
Properties are subject_id,student_id,class_id

So my usecase is i want to find the one student where class_id, student_id matches. Subject id is optional. Means if subject id is present with that student and class id , then pick that student where all 3 id : class id,student id and subject id matches (because it is priority). Else pick the student where student id and class id matches, subject id might be different.

Ps: there might be multiple documents with same class id and student id. If we include subject id then it identify one document.

I tried this one

studentsubject.findone({
    student_id: studentId,
    class_id : classId,
    $or:[
        {
            subject_id : subjectId
        },
        {
            subject_id : {$exists:true}
        }
    ]
})

It fetches the first matched document where class id and student id matches, not looking for existing data where subject id matches

Tags:

2

Answers


  1. 0

    The simplest solution would be to use a JS script. This one should work for your example:

    db = db.getSiblingDB('YOURDB');

const results = db.studentsubject.findOne({ student_id: studentId, class_id: classId, subject_id: subjectId });

if (results) {
  print(results);
} else {
  print(db.events.findOne({ student_id: studentId, class_id: classId, subject_id: { $exists: true } }));
}

    Then you can run this script by using load('path/to/script.js'). Obviously, you need to provide the values, because in this script studentId, classId and subjectId are undefined.

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  2. 0

    It fetches the first matched document where class id and student id matches, not looking for existing data where subject id matches

    Its because you are giving an or statement subject_id : {$exists:true}. your query states:

    Match student_id, match class_id, and match subject_id

    OR

    Match student_id, match class_id, and just check if the document is assigned a subject_id irrespective of its value

    Solution

    Just remove subject_id : {$exists:true}. Your query should look like this:

    db.demo.findOne({
    student_id: "1",
    class_id : "A",
    subject_id : "English"
})

    Explanation

    Lets take this sample data:

    [
  {
    "student_id": "1",
    "class_id": "A",
    "subject_id": "Math",
    "student_name": "Tony"
  },
  {
    "student_id": "2",
    "class_id": "B",
    "subject_id": "English",
    "student_name": "Natasha"
  },
  {
    "student_id": "1",
    "class_id": "A",
    "subject_id": "Science",
    "student_name": "Tony"
  }
]

    Let this be your query:

    db.demo.findOne({
    student_id: "1",
    class_id : "A",
    $or:[
        {
            subject_id : "English"
        },
        {
            subject_id : {$exists:true}
        }
    ]
})

    This will check for student_id: 1, class_id: A, and subject_id: English OR student_id: 1, class_id: A, and if there is any subject_id assigned. So it iterates through the document and stumbles upon this first:

    {
    "student_id": "1",
    "class_id": "A",
    "subject_id": "Math",
    "student_name": "Tony"
}

    The student_id and class_id matches, and subject_id : {$exists:true} as a subject_id does exist there, hence returning this document.

    So modify your query to this:

    db.demo.findOne({
    student_id: "1",
    class_id : "A",
    subject_id : "English"
})

    This will check if all three matches, and don’t worry if a document doesn’t have subject_id field, the query will skip it as it doesn’t meet all the requirements.

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