$unset: 

In MongoDB's Mongo shell, the $unset operator is used to remove a specific field or fields from a document. 

It allows you to unset or delete fields within a document, effectively removing them from the document's structure.


The basic syntax of $unset in the Mongo shell is as follows:

db.collection.updateOne(
   { <query> },
   { $unset: { <field1>: "", <field2>: "" } }
)

Let's break down the syntax:




db.collection.updateOne: This is the method to update a single document in a collection.

{ <query> }: The query parameter specifies the document or set of documents to update. It acts as a filter to match the document(s) you want to modify.

{ $unset: { <field1>: "", <field2>: "" } }: This is the update parameter. The $unset operator is used here, followed by an object where the field(s) to be removed are specified as keys. The values associated with the fields are set to an empty string "".

When the update operation is executed, the specified fields in the matched document(s) will be removed, resulting in a modified document.


Here's an example to illustrate how $unset works:


Consider a collection called users with the following document:

{
   _id: ObjectId("6123456789abcdef01234567"), 
name: "John Doe",
age: 30, 
email: "john@example.com"
}

To remove the email field from this document using $unset, you can run the following command:

db.users.updateOne( 
 { _id: ObjectId("6123456789abcdef01234567") },
 { $unset: { email: "" } }
 )

After executing this command, the updated document will look like this:

{
   _id: ObjectId("6123456789abcdef01234567"), 
name: "John Doe",
age: 30
}

As you can see, the email field has been successfully removed from the document.

Note: If you want to remove multiple fields, you can simply add additional key-value pairs inside the $unset object.

$unset In Aggregation:-

 In the context of MongoDB's Aggregation Framework, the $unset operator is not available. It is specifically used in the update operations in the Mongo shell.


In the Aggregation Framework, there is no direct operator to remove fields from documents. 

The Aggregation Framework focuses on data transformation and aggregation rather than modifying the original documents. 

However, you can achieve similar results by using other operators and stages in the Aggregation Framework.


If you want to exclude certain fields from the output of an aggregation pipeline, you can use the $project stage with the exclusion operator $unset inside it. Here's an example:

db.collection.aggregate([
 {
    $project: { 
field1: 1,
      field2: 1,
      field3: 1,
      fieldToRemove: 
{ $unset: "$fieldToRemove" }
    }
  }
])

In this example, the $project stage is used to include specific fields (field1, field2, field3) while excluding ($unset) the fieldToRemove from the output.



The $project stage allows you to reshape documents, include or exclude fields, create computed fields, and perform various transformations during the aggregation pipeline.

Remember that the $project stage constructs the output documents, so any fields not explicitly included will be excluded by default.

Keep in mind that the $project stage doesn't modify the original documents in the collection; it only affects the shape and content of the resulting documents in the aggregation output.