find, update and return an array object inside a document (mongodb)

Question

I have a bunch of documents in the following format:

{
    _id: "5b38c85f213751038ff4d0bb",
    classes: [
      {
        _id: "5b38c85f21375103ytg65ju7",
        standard: "12",
        numberOfStudents: 45
      },
      {
        _id: "5b38c85f2137510370987hgd",
        standard: "11",
        numberOfStudents: 51
      },
    ]
}

I want to:

Search the collection of documents by the _id and a particular class's _id.
Update the numberOfStudents for that class.
Return only the class object.

So for _id: "5b38c85f213751038ff4d0bb" & classes._id: "5b38c85f21375103ytg65ju7", updating the numberOfStudents: 43 the result I'm expecting is:

{
  _id: "5b38c85f21375103ytg65ju7",
  standard: "12",
  numberOfStudents: 43
}

I've tried a couple of things but they all return the whole document but not the way I want it to be.

It'll be a user defined value so it could be any number. We just have to update it on the basis of the two ids. — m-ketan
– m-ketan, Commented Jul 2, 2018 at 7:19

Ashh · Accepted Answer · 2018-07-02 07:28:49Z

1

You can try below aggregation

db.collection.aggregate([
  { "$match": { "_id": "5b38c85f213751038ff4d0bb" }},
  { "$addFields": {
    "classes": {
      "$filter": {
        "input": "$classes",
        "as": "class",
        "cond": {
          "$eq": [
            "$$class._id",
            "5b38c85f21375103ytg65ju7"
          ]
        }
      }
    }
  }},
  { "$unwind": "$classes" },
  { "$replaceRoot": { "newRoot": "$classes" }},
  { "$project": {
    "_id": 1,
    "numberOfStudents": { "$literal": 43 },
    "standard": 1
  }}
])

Output

[
  {
    "_id": "5b38c85f21375103ytg65ju7",
    "numberOfStudents": 43,
    "standard": "12"
  }
]

Try it here

edited Jul 2, 2018 at 7:28

answered Jul 2, 2018 at 7:23

Ashh

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9 Comments

m-ketan Over a year ago

So I went with your approach and it returns a blank array.

Ashh Over a year ago

I have given you the try section... please have a look

m-ketan Over a year ago

Yeah thats the strange part that it works on the playground but not on my machine.

Ashh Over a year ago

what is your mongodb version ?

Ashh Over a year ago

Then it should work and it should work for mongoose as well... I think you are doing a minor mistake somewhere in the code either is could be with database or something else... try to find it

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