$project (aggregation)

Include Existing Fields

• The _id field is, by default, included in the output documents. To include any other fields from the input documents in the output documents, you must explicitly specify the inclusion in $project.

• If you specify an inclusion of a field that does not exist in the document, $project ignores that field inclusion and does not add the field to the document.

Suppress the _id Field

By default, the _id field is included in the output documents. To exclude the _id field from the output documents, you must explicitly specify the suppression of the _id field in $project.

Exclude Fields

If you specify the exclusion of a field or fields, all other fields are returned in the output documents.

{ $project: { "<field1>": 0, "<field2>": 0, ... } } // Return all but the specified fields

If you specify the exclusion of a field other than _id, you cannot employ any other $project specification forms: i.e. if you exclude fields, you cannot also specify the inclusion of fields, reset the value of existing fields, or add new fields. This restriction does not apply to conditional exclusion of a field using the REMOVE variable.

See also the $unset stage to exclude fields.

Add New Fields or Reset Existing Fields

To add a new field or to reset the value of an existing field, specify the field name and set its value to some expression. For more information on expressions, see Expression Operators.

Embedded Document Fields

When projecting or adding/resetting a field within an embedded document, you can either use dot notation, as in

"contact.address.country": <1 or 0 or expression>

Or you can nest the fields:

contact: { address: { country: <1 or 0 or expression> } }

When nesting the fields, you cannot use dot notation inside the embedded document to specify the field, e.g. contact: { "address.country": <1 or 0 or expression> } is invalid.

{ $project: { contact: 1, "contact.address.country": 1 } }

{ $project: { "contact.address.country": 1, contact: 1 } }

$project Stage Placement

When you use a $project stage it should typically be the last stage in your pipeline, used to specify which fields to return to the client.

Using a $project stage at the beginning or middle of a pipeline to reduce the number of fields passed to subsequent pipeline stages is unlikely to improve performance, as the database performs this optimization automatically.

Restrictions

An error is returned if the $project specification is an empty document.

You cannot use an array index with the $project stage. See Array Indexes are Unsupported.

Include Specific Fields in Output Documents

Consider a books collection with the following document:

{
  "_id" : 1,
  title: "abc123",
  isbn: "0001122223334",
  author: { last: "zzz", first: "aaa" },
  copies: 5
}

The following $project stage includes only the _id, title, and the author fields in its output documents:

db.books.aggregate( [ { $project : { title : 1 , author : 1 } } ] )

The operation results in the following document:

{ "_id" : 1, "title" : "abc123", "author" : { "last" : "zzz", "first" : "aaa" } }

Suppress _id Field in the Output Documents

The _id field is always included by default. To exclude the _id field from the output documents of the $project stage, specify the exclusion of the _id field by setting it to 0 in the projection document.

Consider a books collection with the following document:

{
  "_id" : 1,
  title: "abc123",
  isbn: "0001122223334",
  author: { last: "zzz", first: "aaa" },
  copies: 5
}

The following $project stage excludes the _id field but includes the title, and the author fields in its output documents:

db.books.aggregate( [ { $project : { _id: 0, title : 1 , author : 1 } } ] )

The operation results in the following document:

{ "title" : "abc123", "author" : { "last" : "zzz", "first" : "aaa" } }

Exclude Fields from Output Documents

Consider a books collection with the following document:

{
  "_id" : 1,
  title: "abc123",
  isbn: "0001122223334",
  author: { last: "zzz", first: "aaa" },
  copies: 5,
  lastModified: "2016-07-28"
}

The following $project stage excludes the lastModified field from the output:

db.books.aggregate( [ { $project : { "lastModified": 0 } } ] )

See also the $unset stage to exclude fields.

Exclude Fields from Embedded Documents

Consider a books collection with the following document:

{
  "_id" : 1,
  title: "abc123",
  isbn: "0001122223334",
  author: { last: "zzz", first: "aaa" },
  copies: 5,
  lastModified: "2016-07-28"
}

The following $project stage excludes the author.first and lastModified fields from the output:

db.books.aggregate( [ { $project : { "author.first" : 0, "lastModified" : 0 } } ] )

Alternatively, you can nest the exclusion specification in a document:

db.bookmarks.aggregate( [ { $project: { "author": { "first": 0}, "lastModified" : 0 } } ] )

Both specifications result in the same output:

{
   "_id" : 1,
   "title" : "abc123",
   "isbn" : "0001122223334",
   "author" : {
      "last" : "zzz"
   },
   "copies" : 5,
}

See also the $unset stage to exclude fields.

Conditionally Exclude Fields

You can use the variable REMOVE in aggregation expressions to conditionally suppress a field.

Consider a books collection with the following document:

{
  "_id" : 1,
  title: "abc123",
  isbn: "0001122223334",
  author: { last: "zzz", first: "aaa" },
  copies: 5,
  lastModified: "2016-07-28"
}
{
  "_id" : 2,
  title: "Baked Goods",
  isbn: "9999999999999",
  author: { last: "xyz", first: "abc", middle: "" },
  copies: 2,
  lastModified: "2017-07-21"
}
{
  "_id" : 3,
  title: "Ice Cream Cakes",
  isbn: "8888888888888",
  author: { last: "xyz", first: "abc", middle: "mmm" },
  copies: 5,
  lastModified: "2017-07-22"
}

The following $project stage uses the REMOVE variable to excludes the author.middle field only if it equals "":

db.books.aggregate( [
   {
      $project: {
         title: 1,
         "author.first": 1,
         "author.last" : 1,
         "author.middle": {
            $cond: {
               if: { $eq: [ "", "$author.middle" ] },
               then: "$$REMOVE",
               else: "$author.middle"
            }
         }
      }
   }
] )

The aggregation operation results in the following output:

{ "_id" : 1, "title" : "abc123", "author" : { "last" : "zzz", "first" : "aaa" } }
{ "_id" : 2, "title" : "Baked Goods", "author" : { "last" : "xyz", "first" : "abc" } }
{ "_id" : 3, "title" : "Ice Cream Cakes", "author" : { "last" : "xyz", "first" : "abc", "middle" : "mmm" } }

Include Specific Fields from Embedded Documents

Consider a bookmarks collection with the following documents:

{ _id: 1, user: "1234", stop: { title: "book1", author: "xyz", page: 32 } }
{ _id: 2, user: "7890", stop: [ { title: "book2", author: "abc", page: 5 }, { title: "book3", author: "ijk", page: 100 } ] }

To include only the title field in the embedded document in the stop field, you can use the dot notation:

db.bookmarks.aggregate( [ { $project: { "stop.title": 1 } } ] )

Or, you can nest the inclusion specification in a document:

db.bookmarks.aggregate( [ { $project: { stop: { title: 1 } } } ] )

Both specifications result in the following documents:

{ "_id" : 1, "stop" : { "title" : "book1" } }
{ "_id" : 2, "stop" : [ { "title" : "book2" }, { "title" : "book3" } ] }

Include Computed Fields

Consider a books collection with the following document:

{
  "_id" : 1,
  title: "abc123",
  isbn: "0001122223334",
  author: { last: "zzz", first: "aaa" },
  copies: 5
}

The following $project stage adds the new fields isbn, lastName, and copiesSold:

db.books.aggregate(
   [
      {
         $project: {
            title: 1,
            isbn: {
               prefix: { $substr: [ "$isbn", 0, 3 ] },
               group: { $substr: [ "$isbn", 3, 2 ] },
               publisher: { $substr: [ "$isbn", 5, 4 ] },
               title: { $substr: [ "$isbn", 9, 3 ] },
               checkDigit: { $substr: [ "$isbn", 12, 1] }
            },
            lastName: "$author.last",
            copiesSold: "$copies"
         }
      }
   ]
)

The operation results in the following document:

{
   "_id" : 1,
   "title" : "abc123",
   "isbn" : {
      "prefix" : "000",
      "group" : "11",
      "publisher" : "2222",
      "title" : "333",
      "checkDigit" : "4"
   },
   "lastName" : "zzz",
   "copiesSold" : 5
}

Project New Array Fields

For example, if a collection includes the following document:

{ "_id" : ObjectId("55ad167f320c6be244eb3b95"), "x" : 1, "y" : 1 }

The following operation projects the fields x and y as elements in a new field myArray:

db.collection.aggregate( [ { $project: { myArray: [ "$x", "$y" ] } } ] )

The operation returns the following document:

{ "_id" : ObjectId("55ad167f320c6be244eb3b95"), "myArray" : [ 1, 1 ] }

If array specification includes fields that are non-existent in a document, the operation substitutes null as the value for that field.

For example, given the same document as above, the following operation projects the fields x, y, and a non-existing field $someField as elements in a new field myArray:

db.collection.aggregate( [ { $project: { myArray: [ "$x", "$y", "$someField" ] } } ] )

The operation returns the following document:

{ "_id" : ObjectId("55ad167f320c6be244eb3b95"), "myArray" : [ 1, 1, null ] }

Array Indexes are Unsupported

You cannot use an array index with the $project stage. This section shows an example.

Create the following pizzas collection:

db.pizzas.insert( [
   { _id: 0, name: [ 'Pepperoni' ] },
] )

The following example returns the pizza:

db.pizzas.aggregate( [
   { $project: { x: '$name', _id: 0 } },
] )

The pizza is returned in the example output:

[ { x: [ 'Pepperoni' ] } ]

The following example uses an array index ($name.0) to attempt to return the pizza:

db.pizzas.aggregate( [
   { $project: { x: '$name.0', _id: 0 } },
] )

The pizza is not returned in the example output:

[ { x: [] } ]