How to remove key fields in MongoDB?

To remove key fields in MongoDB, you can use the $unset operator. This operator deletes specified fields from documents in a collection.

Syntax

db.collection.updateMany(
    { query },
    { $unset: { fieldName: 1 } }
);

Sample Data

Let us first create a collection with sample documents ?

db.removeKeyFieldsDemo.insertMany([
    {
        "StudentFirstName": "John",
        "StudentLastName": "Doe",
        "StudentAge": 23
    },
    {
        "StudentFirstName": "John",
        "StudentLastName": "Smith",
        "StudentAge": 21
    }
]);

{
   "acknowledged": true,
   "insertedIds": [
      ObjectId("5cc6c8289cb58ca2b005e672"),
      ObjectId("5cc6c8359cb58ca2b005e673")
   ]
}

View Sample Data

db.removeKeyFieldsDemo.find();

{
   "_id": ObjectId("5cc6c8289cb58ca2b005e672"),
   "StudentFirstName": "John",
   "StudentLastName": "Doe",
   "StudentAge": 23
}
{
   "_id": ObjectId("5cc6c8359cb58ca2b005e673"),
   "StudentFirstName": "John",
   "StudentLastName": "Smith",
   "StudentAge": 21
}

Remove Key Fields

Now let's remove the StudentAge field from all documents ?

db.removeKeyFieldsDemo.updateMany(
    {},
    { $unset: { StudentAge: 1 } }
);

{
   "acknowledged": true,
   "matchedCount": 2,
   "modifiedCount": 2
}

Verify Result

db.removeKeyFieldsDemo.find();

{
   "_id": ObjectId("5cc6c8289cb58ca2b005e672"),
   "StudentFirstName": "John",
   "StudentLastName": "Doe"
}
{
   "_id": ObjectId("5cc6c8359cb58ca2b005e673"),
   "StudentFirstName": "John",
   "StudentLastName": "Smith"
}

Key Points

• The value 1 in $unset is just a placeholder; any value works.
• Use an empty query {} to remove fields from all documents.
• Add specific conditions to target only certain documents.

Conclusion

The $unset operator efficiently removes unwanted fields from MongoDB documents. Combine it with updateMany() to remove fields from multiple documents at once.