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$ (update)

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$

The positional $ operator identifies an element in an array to update without explicitly specifying the position of the element in the array.

Note
Disambiguation
  • To project, or return, an array element from a read operation, see the $ projection operator instead.
  • To update all elements in an array, see the all positional operator $[] instead.
  • To update all elements that match an array filter condition or conditions, see the filtered positional operator instead $[<identifier>].

The positional $ operator has the form:

{ "<array>.$" : value }

When used with update operations, e.g. db.collection.update() and db.collection.findAndModify(),

  • 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.

For example:

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

Starting in MongoDB 5.0, update operators process document fields with string-based names in lexicographic order. Fields with numeric names are processed in numeric order. See Update Operators Behavior for details.

Do not use the positional operator $ with upsert operations because inserts will use the $ as a field name in the inserted document.

The positional $ operator cannot be used for queries which traverse more than one array, such as queries that traverse arrays nested within other arrays, because the replacement for the $ placeholder is a single value

When used with the $unset operator, the positional $ operator does not remove the matching element from the array but rather sets it to null.

If the query matches the array using a negation operator, such as $ne, $not, or $nin, then you cannot use the positional operator to update values from this array.

However, if the negated portion of the query is inside of an $elemMatch expression, then you can use the positional operator to update this field.

Create a collection students with the following documents:

db.students.insert([
{ "_id" : 1, "grades" : [ 85, 80, 80 ] },
{ "_id" : 2, "grades" : [ 88, 90, 92 ] },
{ "_id" : 3, "grades" : [ 85, 100, 90 ] }
])

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

Important

You must include the array field as part of the query document.

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

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

After the operation, the students collection contains the following documents:

{ "_id" : 1, "grades" : [ 85, 82, 80 ] }
{ "_id" : 2, "grades" : [ 88, 90, 92 ] }
{ "_id" : 3, "grades" : [ 85, 100, 90 ] }

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.

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

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

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

Use the positional $ operator to update the std field of the first array element that matches the grade equal to 85 condition:

Important

You must include the array field as part of the query document.

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

After the operation, the document has the following updated values:

{
"_id" : 4,
"grades" : [
{ "grade" : 80, "mean" : 75, "std" : 8 },
{ "grade" : 85, "mean" : 90, "std" : 6 },
{ "grade" : 85, "mean" : 85, "std" : 8 }
]
}

The $ operator can update the first array element that matches multiple query criteria specified with the $elemMatch operator.

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

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

In the example below, the $ operator updates the value of the std field in the first embedded document that has grade field with a value less than or equal to 90 and a mean field with a value greater than 80:

db.students.updateOne(
{
_id: 5,
grades: { $elemMatch: { grade: { $lte: 90 }, mean: { $gt: 80 } } }
},
{ $set: { "grades.$.std" : 6 } }
)

This operation updates the first embedded document that matches the criteria, namely the second embedded document in the array:

{
_id: 5,
grades: [
{ grade: 80, mean: 75, std: 8 },
{ grade: 85, mean: 90, std: 6 },
{ grade: 90, mean: 85, std: 3 }
]
}
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