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I am working in a MERN project and i want to fetch data from inside the array of objects through the inside objectid.

My model view

enter image description here

I want to fetch name:"vikash" and attendance:"present" through the objectid:"63cbb3b15cd59c7810ab89a2

I had tried two types of methods from previous stackoverfow solution but they didn't work for me

app.get('/stud/:id', (req, res) => {
    ProjectSchema.find({"projectmembers._id":req.params._id}, (err, data) => {
        if (err) return res.status(500).send(err);
        res.status(200).send(data);
    });
})

the second one is

app.get('/stu/:id', (req, res) => {
    ProjectSchema.aggregate([
        {
            $unwind:"$projectmembers"
        },
        {$match : {
            "projectmembers._id" : req.params._id
        }}
    ], (err, data) => {
        if (err) return res.status(500).send(err);
        res.status(200).send(data);
    });
})

The output of both code is []

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1 Answer 1

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1

There was also a typo in your code. The params name is id, so it should be req.params.id and not req.params._id - that's probably the reason why you were receiving an empty array.

For your case, you can filter the result so that you only receive the project member of a specific id through aggregation.

For example:

    app.get('/stu/:id', (req, res) => {
    
        try {
          const student = await ProjectSchema.aggregate([
  {
    $project: {
      _id: 0,
      projectmembers: {
        $filter: {
          input: "$projectmembers",
          as: "projectMember",
          cond: {
            $eq: [
              "$$projectMember._id",
              ObjectId(req.params.id)
            ]
          }
        }
      }
    }
  }
]);

      if (!student)
        return res.status(400).json({
          success: false,
          message: "No student exists with this ID",
        });
        
      return res.status(200).json({
        success: true,
        message: "Student Found",
        studentDetails: student,
      });
    } catch (err) {
      return res.status(500).json({
        success: false,
        message: "Error encountered whilst finding student",
        error: err,
      });
    }
  });

You can see that there is a $project stage - what this does is tells mongo what you want to return. I wrote _id: 0, which tells mongo that you do not want to return the document's _id field. Then the $filter stage has an input field, which tells mongo what to filter - in our case projectmembers. In the $as field is the name of the variable for each item in your projectmembers array, which I named projectMember - you could name it anything you like. Then we can access each item via the variable named projectMember in our condition statement cond we passed an $eq condition, which checks the projectmembers array for any $$projectMember._id (in the eq condition, we prefix projectMember.id with two $$ in order to access it) which is equal to req.params.id.

To access the ObjectId, you will need to import it at the top of your file:

const { ObjectId } = require("mongodb");

Finally, you can also see how the mongo aggregation works here at Mongo Playground and play around with it.

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6 Comments

@scot...thanks for your answer..actually i am new to mongo so..... i got the output is { "success": true, "message": "Student Found", "studentDetails": [] }.... how to retrieve the backend data
No problem. Have you console logged req.params.id to make sure you are receiving the correct value?
yes i am getting the id
can you please console.log student after the await call and see what the output is?
i got like this []
|

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