Mongoose get objects which match an object item from an array of objects

Question

I would like to find a single document matching the courseID but inside the document only objects from the materials array whose moduleNo matches the one I give. But the query I currently use seems to return the correct document but also returns all the objects in materials array. Any help would be appreciated.

My schema,

const materialSchema = new mongoose.Schema({
  courseID: String,
  materials: [
    {
      moduleNo: Number,
      moduleMaterial: String,
    },
  ],
});

My code/query,

 exports.getMaterials = (req, res) => {
  const { courseID, moduleNo } = req.query;
  Material.findOne(
    { courseID, "materials.moduleNo": moduleNo },
    (err, result) => {
      if (err) {
        console.error(err);
      } else {
        res.json(result);
      }
    }
  );
};

Saurabh Mistry · Accepted Answer · 2022-06-26 16:21:39Z

3

Way 1 : Use $elemMatch operator in project field

The $elemMatch operator limits the contents of an array field from the query results to contain only the first element matching the $elemMatch condition

Result : Returns only one matching array element

syntax :

db.collection.find(query,projection)

db.collection.find({
     "field": field_value                  
  },{
    "array_name":{
       $elemMatch:{"key_name": value }
    },
    field:1,
    field_2:1,
    field_3:0
})

https://mongoplayground.net/p/HKT1Gop32Pq

Way 2 : Array Field Limitations array.$ in project field *

Result : Returns only one matching array element

db.collection.find({
     "field": field_value,
     "array_name.key_name": value       
  },{
        "array_name.$":1,
        field:1,
        field_2:1,
        field_3:0
 });

https://mongoplayground.net/p/Db0azCakQg9

Update : Using MongoDB Aggregation

Result : Returns multiple matching array elements

db.collection.aggregate([
  {
    "$unwind": "$materials"
  },
  {
    "$match": {
      "courseID": "Demo",
      "materials.moduleNo": 1
    }
  }
])

will return output as :

[
  {
    "_id": ObjectId("5a934e000102030405000000"),
    "courseID": "Demo",
    "materials": {
      "moduleMaterial": "A",
      "moduleNo": 1
    }
  },
  {
    "_id": ObjectId("5a934e000102030405000000"),
    "courseID": "Demo",
    "materials": {
      "moduleMaterial": "B",
      "moduleNo": 1
    }
  }
]

And If you want to format output :

db.collection.aggregate([
  {
    "$unwind": "$materials"
  },
  {
    "$match": {
      "courseID": "Demo",
      "materials.moduleNo": 1
    }
  },
  {
    "$group": {
      "_id": {
        "courseID": "$courseID",
        "moduleNo": "$materials.moduleNo"
      },
      "materials": {
        "$push": "$materials.moduleMaterial"
      }
    }
  },
  {
    "$project": {
      "_id": 0,
      "courseID": "$_id.courseID",
      "moduleNo": "$_id.moduleNo",
      "materials": "$materials"
    }
  }
])

will return result as :

[
  {
    "courseID": "Demo",
    "materials": [
      "A",
      "B"
    ],
    "moduleNo": 1
  }
]

https://mongoplayground.net/p/vdPVbce6WkX

edited Jun 26, 2022 at 16:21

answered Jun 26, 2022 at 3:56

Saurabh Mistry

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2 Comments

Jonan Mathew George Over a year ago

Thanks for this but there might be more than one materials object with same moduleNo, but this only returns one object back. Any solution for that?

Saurabh Mistry Over a year ago

@JonanMathewGeorge , Thanks for update , please check updated answer.

Charchit Kapoor · Accepted Answer · 2022-06-26 15:45:39Z

1

Use the $elemMatch operator, something lik this:

exports.getMaterials = (req, res) => {
  const { courseID, moduleNo } = req.query;
  Material.findOne(
    { courseID },
    {"materials": { $elemMatch: {moduleNo: moduleNo}},
    (err, result) => {
      if (err) {
        console.error(err);
      } else {
        res.json(result);
      }
    }
  );
};

Update: To return all matching elements in the array, you will have to use an aggregation pipeline, having $filter stage, to filter out array elements. Like this:

exports.getMaterials = (req, res) => {
  const { courseID, moduleNo } = req.query;
  Material.aggregate(
    [
  {
    $match: {
      courseID: courseID
    }
  },
  {
    "$project": {
      courseID: 1,
      materials: {
        "$filter": {
          "input": "$materials",
          "as": "material",
          "cond": {
            "$eq": [
              "$$material.moduleNo",
              moduleNo
            ]
          }
        }
      }
    }
  }
]
  );
};

Here's the playground link.

edited Jun 26, 2022 at 15:45

answered Jun 26, 2022 at 3:04

Charchit Kapoor

9,3242 gold badges11 silver badges26 bronze badges

1 Comment

Jonan Mathew George Over a year ago

This works but returns only one object but there might be more than one object with same moduleNo. Is there any solution for that.

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