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Aggregation Pipeline Stages

$redact (aggregation)

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  • Definition
  • Examples

Definition

$redact

Restricts the contents of the documents based on information stored in the documents themselves.

Diagram of security architecture with middleware and redaction.

The $redact stage has the following prototype form:

{ $redact: <expression> }

The argument can be any valid expression as long as it resolves to the $$DESCEND, $$PRUNE, or $$KEEP system variables. For more information on expressions, see Expressions.

System Variable
Description
$$DESCEND
$redact returns the fields at the current document level, excluding embedded documents. To include embedded documents and embedded documents within arrays, apply the $cond expression to the embedded documents to determine access for these embedded documents.
$$PRUNE
$redact excludes all fields at this current document/embedded document level, without further inspection of any of the excluded fields. This applies even if the excluded field contains embedded documents that may have different access levels.
$$KEEP
$redact returns or keeps all fields at this current document/embedded document level, without further inspection of the fields at this level. This applies even if the included field contains embedded documents that may have different access levels.

Examples

The examples in this section use the db.collection.aggregate() helper.

Evaluate Access at Every Document Level

A forecasts collection contains documents of the following form where the tags field lists the different access values for that document/embedded document level; i.e. a value of [ "G", "STLW" ] specifies either "G" or "STLW" can access the data:

{
  _id: 1,
  title: "123 Department Report",
  tags: [ "G", "STLW" ],
  year: 2014,
  subsections: [
    {
      subtitle: "Section 1: Overview",
      tags: [ "SI", "G" ],
      content:  "Section 1: This is the content of section 1."
    },
    {
      subtitle: "Section 2: Analysis",
      tags: [ "STLW" ],
      content: "Section 2: This is the content of section 2."
    },
    {
      subtitle: "Section 3: Budgeting",
      tags: [ "TK" ],
      content: {
        text: "Section 3: This is the content of section3.",
        tags: [ "HCS" ]
      }
    }
  ]
}

A user has access to view information with either the tag "STLW" or "G". To run a query on all documents with year 2014 for this user, include a $redact stage as in the following:

var userAccess = [ "STLW", "G" ];
db.forecasts.aggregate(
   [
     { $match: { year: 2014 } },
     { $redact: {
        $cond: {
           if: { $gt: [ { $size: { $setIntersection: [ "$tags", userAccess ] } }, 0 ] },
           then: "$$DESCEND",
           else: "$$PRUNE"
         }
       }
     }
   ]
);

The aggregation operation returns the following "redacted" document:

{
  "_id" : 1,
  "title" : "123 Department Report",
  "tags" : [ "G", "STLW" ],
  "year" : 2014,
  "subsections" : [
    {
      "subtitle" : "Section 1: Overview",
      "tags" : [ "SI", "G" ],
      "content" : "Section 1: This is the content of section 1."
    },
    {
      "subtitle" : "Section 2: Analysis",
      "tags" : [ "STLW" ],
      "content" : "Section 2: This is the content of section 2."
    }
  ]
}

Tip

See also:

  • $size

  • $setIntersection

Exclude All Fields at a Given Level

A collection accounts contains the following document:

{
  _id: 1,
  level: 1,
  acct_id: "xyz123",
  cc: {
    level: 5,
    type: "yy",
    num: 000000000000,
    exp_date: ISODate("2015-11-01T00:00:00.000Z"),
    billing_addr: {
      level: 5,
      addr1: "123 ABC Street",
      city: "Some City"
    },
    shipping_addr: [
      {
        level: 3,
        addr1: "987 XYZ Ave",
        city: "Some City"
      },
      {
        level: 3,
        addr1: "PO Box 0123",
        city: "Some City"
      }
    ]
  },
  status: "A"
}

In this example document, the level field determines the access level required to view the data.

To run a query on all documents with status A and exclude all fields contained in a document/embedded document at level 5, include a $redact stage that specifies the system variable "$$PRUNE" in the then field:

db.accounts.aggregate(
  [
    { $match: { status: "A" } },
    {
      $redact: {
        $cond: {
          if: { $eq: [ "$level", 5 ] },
          then: "$$PRUNE",
          else: "$$DESCEND"
        }
      }
    }
  ]
);

The $redact stage evaluates the level field to determine access. If the level field equals 5, then exclude all fields at that level, even if the excluded field contains embedded documents that may have different level values, such as the shipping_addr field.

The aggregation operation returns the following "redacted" document:

{
  "_id" : 1,
  "level" : 1,
  "acct_id" : "xyz123",
  "status" : "A"
}

The result set shows that the $redact stage excluded the field cc as a whole, including the shipping_addr field which contained embedded documents that had level field values equal to 3 and not 5.

Tip

See also:

Implement Field Level Redaction for steps to set up multiple combinations of access for the same data.

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$project (aggregation)

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