MongoDB query to add a document in an already created collection

To add a document to an existing collection in MongoDB, use the $push operator. This operator appends new documents to an existing array field within a document.

Syntax

db.collection.update(
   { "field": "value" },
   { $push: { "arrayField": { newDocument } } }
);

Sample Data

Let's create a collection with sample documents ?

db.demo177.insertOne({
   "Id": "101",
   "details": [
      {
         "StudentName": "Chris",
         "Scores": [67, 71, 74],
         "SubjectName": ["MySQL", "Java"]
      },
      {
         "StudentName": "David",
         "Scores": [89, 98, 45],
         "SubjectName": ["PL/SQL", "C"]
      }
   ]
});

{
   "acknowledged": true,
   "insertedId": ObjectId("5e384b2b9e4f06af551997f4")
}

Display Current Documents

db.demo177.find().pretty();

{
   "_id": ObjectId("5e384b2b9e4f06af551997f4"),
   "Id": "101",
   "details": [
      {
         "StudentName": "Chris",
         "Scores": [67, 71, 74],
         "SubjectName": ["MySQL", "Java"]
      },
      {
         "StudentName": "David",
         "Scores": [89, 98, 45],
         "SubjectName": ["PL/SQL", "C"]
      }
   ]
}

Example: Adding Document to Array

Following is the query to add a document to the existing details array ?

db.demo177.update(
   { "Id": "101" },
   {
      $push: {
         "details": {
            "StudentName": "Bob",
            "Scores": [90, 91, 94],
            "SubjectName": ["MongoDB", "SQL Server"]
         }
      }
   }
);

WriteResult({ "nMatched": 1, "nUpserted": 0, "nModified": 1 })

Verify Result

db.demo177.find().pretty();

{
   "_id": ObjectId("5e384b2b9e4f06af551997f4"),
   "Id": "101",
   "details": [
      {
         "StudentName": "Chris",
         "Scores": [67, 71, 74],
         "SubjectName": ["MySQL", "Java"]
      },
      {
         "StudentName": "David",
         "Scores": [89, 98, 45],
         "SubjectName": ["PL/SQL", "C"]
      },
      {
         "StudentName": "Bob",
         "Scores": [90, 91, 94],
         "SubjectName": ["MongoDB", "SQL Server"]
      }
   ]
}

Key Points

• $push appends the new document to the end of the array field
• The update operation modifies the matching document in-place
• Use a proper query filter to target the specific document you want to modify

Conclusion

The $push operator is the primary method to add documents to array fields in existing MongoDB collections. It provides a clean way to append new data without replacing existing array contents.