Using $redact in Mongoose Aggregation(Simple example)

$REDACT EXCLUDES ALL FIELDS AT THIS CURRENT DOCUMENT/EMBEDDED DOCUMENT LEVEL, WITHOUT FURTHER INSPECTION OF ANY OF THE EXCLUDED FIELDS.
RESOURCE: $redact (aggregation) — MongoDB Manual

The $redact stage in MongoDB aggregation allows you to restrict entire documents or content within documents from being outputted based on information stored in the documents themselves. It’s a powerful tool for controlling the visibility of data based on specific conditions.

Scenario

Suppose we have a collection of users, and we want to retrieve specific information about users named “sohrab.” We’ll demonstrate how to use $redact to achieve this.

User Data

Let’s consider the following sample user data:

const Users = [
  {
    "id": 1,
    "name": "john",
    "age": 30,
    "phone": "[phone]"
  },
  {
    "id": 2,
    "name": "tom",
    "age": 25,
    "phone": "[phone]"
  },
  {
    "id": 3,
    "name": "sohrab",
    "age": 23,
    "phone": "[phone]"
  },
  {
    "id": 4,
    "name": "sohrab",
    "age": 25,
    "phone": "[phone]"
  }
];

Step 1: Using $match

We start by using the $match stage to filter users with the name “sohrab”:

Users.aggregate([
  {
    $match: {
      name: "sohrab"
    }
  }
]);

The results will include the following documents:

User with id = 3:
Age: 23
Name: “sohrab”
Phone: “[phone]”
User with id = 4:
Age: 25
Name: “sohrab”
Phone: “[phone]”

Step 2: Using $redact and $cond

Now let’s further limit the output. We want to keep only users with an age of 25. We’ll use $redact and $cond for this purpose:

Users.aggregate([
  {
    $match: {
      name: "sohrab"
    }
  },
  {
    $redact: {
      $cond: {
        if: {
          $eq: ["$age", 25]
        },
        then: "$$DESCEND", // Keep fields at this level
        else: "$$PRUNE" // Exclude fields at this level
      }
    }
  },
  {
    $project: {
      _id: false,
      age: "$age",
      phone: "$phone"
    }
  }
]);

The resulting output will be:

[
  {
    "age": 25,
    "phone": "[phone]"
  }
]

In this final output, we’ve kept only the age and phone fields for users named “sohrab” with an age of 25.

Remember that $redact allows you to control the visibility of fields based on conditions, making it a valuable tool for fine-tuning your aggregation results.