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This article will discuss how the $unset operator works in MongoDB. Additionally, we will demonstrate the use of this operator to remove a field from all documents in a MongoDB collection.

$unset operator in MongoDB

$unset is an operator used to remove a field from an entity. It is used in the update method of MongoDB and will not have any effect if no match is found in the said method.

Suppose you have the following document in a MongoDB database:

> db.employee.find().pretty()
{"_id":1,   "name":"Alice", "salary": 1500}
{"_id":2,   "name":"Bob",   "salary": 2500}
{"_id":3,   "name":"Jhon",  "salary": 3500, "role":"Owner"}
{"_id":4,   "name":"Grace", "salary": 4500}

You have 4 entities in the document and you have used the find() method without parameters. It will return all the entities in the employee collection and pretty() is used to print the entities in a formatted manner.

Now, suppose you don't want to show the field role for some reason. So, you should remove it from the row.

The query should look like this:

> db.employee.update({_id:3}, {$unset : {"role":""}} )
> db.employee.find().pretty()
{"_id":1,   "name":"Alice", "salary": 1500}
{"_id":2,   "name":"Bob",   "salary": 2500}
{"_id":3,   "name":"Jhon",  "salary": 3500}
{"_id":4,   "name":"Grace", "salary": 4500}

Now it will remove the field role from the first matching document where id = 3.

Another option named multi is short for multiple and has a default value of false.

If true, affects update queries for all matching documents.

Use the $unset operator to remove a field from all documents in MongoDB

Suppose you are asked to delete the field salary from all documents in a MongoDB collection. If we use one update command for each id, it will take longer time.

Instead, we should do the following:

> db.employee.update({}, {$unset:{"salary": ""}}, {multi: true})
> db.employee.find().pretty()
{"_id":1,   "name":"Alice"  }
{"_id":2,   "name":"Bob"    }
{"_id":3,   "name":"Jhon"   }
{"_id":4,   "name":"Grace"  }

We used an empty object in the update query and set true in multi. Basically, {} will select all the documents in the object, while {multi: true} affects all the matching documents in the update method.

Here is the basic format (from the official documentation):

db.collection.updateOne(<filter>, <update>, <options>)
db.collection.updateMany(<filter>, <update>, <options>)
db.collection.replaceOne(<filter>, <update>, <options>)

In update methods you can use $unsetthe operator, and in options you can use multi.