How to fetch the list of data from nested json in MongoDb based on key?

Question

I have a JSON in MongoDB with collection name StudentData as:

"data" : {
  "students":{
      "A":{
        "roll":"11",
        "otherDetails":{
            "name":"AAA"
        }
      },
      "B":{
        "roll":"12",
        "otherDetails":{
            "name":"BBB"
        }
      },
      "C":{
        "roll":"13",
        "otherDetails":{
            "name":"CCC"
        }
      },
      "D":{
        "roll":"14",
        "otherDetails":{
            "name":"DDD"
        }
      }
  }
}

How can I fetch all students A, I tried the following db.StudentData.find({"data.students":"A"}) but it is returning nothing .

I am trying to get this:

{
   "roll":"11",
   "otherDetails":{
   "name":"AAA"
  }
}

What could be it's Java Equivalent and Mongo equivalent queries?

MilanRegmi · Accepted Answer · 2019-08-21 06:09:09Z

1

A is object in your data but you are fetching it as a value. So, it gives you nothing. Try this:

db.collection.find({},
{
  "data.students.A": 1
})

answered Aug 21, 2019 at 6:09

MilanRegmi

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4 Comments

Neeraj Nauni Over a year ago

It is somewhat working . What is the meaning of 1 here ??

MilanRegmi Over a year ago

It means that you are projecting the field "data.students.A" only while fetching list of data.

Neeraj Nauni Over a year ago

Thanks . Can you let me know how do I convert it into Java equivalent using MongoDb criteria ?

MilanRegmi Over a year ago

collection.find(new BasicDBObject(), new BasicDBObject("data.students.A", 1));

Hardik Shah · Accepted Answer · 2019-08-21 11:10:17Z

It is a simple approach suggested by @MilanRegmi

Though, I just would like to add other possible ways using aggregation.

db.collections.aggregate([
  { $addFields: { studentsList: { $objectToArray: "$data.students" } } },
  { $match: { "studentsList.k": "A" }},
  { $project: {
    students: { $filter: {
        input: '$studentsList',
        as: 'student',
        cond: { $eq: [ '$$student.k', 'A' ]}
    }},
    _id: 0
  }},
  { $unwind: "$students" }, // Not required if you are ok with having some nested array
  { $replaceRoot: { newRoot: "$students.v" }} // Not required if you are ok with having some nested array
])

And one more optimized query is:

db.collection.aggregate([
  { $addFields: { studentsList: { $objectToArray: "$data.students" } } },
  { $unwind: "$studentsList" },
  { $match: { "studentsList.k": "A" }},
  { $replaceRoot: { newRoot: "$studentsList.v" }} // Not required if you are ok with having object with some other fields
])

Output: (For both queries)

/* 1 */
{
    "roll" : "11",
    "otherDetails" : {
        "name" : "AAA"
    }
}

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