Reference >
Query, Update, Projection, and Aggregation Operators >
$ (update)

$ (update)¶

$¶

Syntax: { "<array>.$" : value }

The positional $ operator identifies an element in an array field to update without explicitly specifying the position of the element in the array. To project, or return, an array element from a read operation, see the $ projection operator.

When used with the update() method,

the positional $ operator acts as a placeholder for the first element that matches the query document, and
the array field must appear as part of the query document.

copy

db.collection.update( { <array>: value ... }, { <update operator>: { "<array>.$" : value } } )

Consider a collection students with the following documents:

copy

{ "_id" : 1, "grades" : [ 80, 85, 90 ] }
{ "_id" : 2, "grades" : [ 88, 90, 92 ] }
{ "_id" : 3, "grades" : [ 85, 100, 90 ] }

To update 80 to 82 in the grades array in the first document, use the positional $ operator if you do not know the position of the element in the array:

copy

db.students.update( { _id: 1, grades: 80 }, { $set: { "grades.$" : 82 } } )

Remember that the positional $ operator acts as a placeholder for the first match of the update query document.

The positional $ operator facilitates updates to arrays that contain embedded documents. Use the positional $ operator to access the fields in the embedded documents with the dot notation on the $ operator.

copy

db.collection.update( { <query selector> }, { <update operator>: { "array.$.field" : value } } )

Consider the following document in the students collection whose grades field value is an array of embedded documents:

copy

{ "_id" : 4, "grades" : [ { grade: 80, mean: 75, std: 8 },
                          { grade: 85, mean: 90, std: 5 },
                          { grade: 90, mean: 85, std: 3 } ] }

Use the positional $ operator to update the value of the std field in the embedded document with the grade of 85:

copy

db.students.update( { _id: 4, "grades.grade": 85 }, { $set: { "grades.$.std" : 6 } } )

Note

Do not use the positional operator $ with upsert operations because inserts will use the $ as a field name in the inserted document.
When used with the $unset operator, the positional $ operator does not remove the matching element from the array but rather sets it to null.