$ (positional update operator)

upsert

Do not use the $ operator in upsert operations. If the update query does not match any existing documents, the upsert fails because the $ operator requires a matching array element.

Nested Arrays

You cannot use the positional $ operator for queries that 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.

Unsets

When you use the positional $ operator with the $unset operator, the positional operator does not remove the matching element from the array. Instead, it sets the element to null.

Negations

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.

You can use the positional operator if the negated portion of the query is inside an $elemMatch expression.

Multiple Array Matches

The positional $ update operator behaves ambiguously when you filter on multiple array fields.

When the server executes an update method, it first runs a query to determine which documents to update. If the update filters documents on multiple array fields, the positional $ update operator might not update the correct position in the array.

For more information, see the example.

Update Values in an Array

Create the students collection with the following documents:

db.students.insertMany( [
   { "_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:

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:

db.students.insertMany ( [
   { _id : 1, "grades" : [ 85, 82, 80 ] },
   { _id : 2, "grades" : [ 88, 90, 92 ] },
   { _id : 3, "grades" : [ 85, 100, 90 ] }
] )

Update Documents in an Array

The positional $ operator allows updates to arrays that have embedded documents. Use the positional $ operator to access fields in embedded documents. Use dot notation on the $ operator.

db.collection.updateOne(
   { <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:

db.students.insertOne( [
   {
      _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:

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 }
   ]
}

Update Embedded Documents Using Multiple Field Matches

Use the $ operator to 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:

db.students.insertOne( [
   {
     _id: 5,
      grades: [
         { grade: 80, mean: 75, std: 8 },
         { grade: 85, mean: 90, std: 5 },
         { grade: 90, mean: 85, std: 3 }
      ]
   }
] )

In the following example, the $ operator updates the value of the std field in the first embedded document that has a 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 }
  ]
}

Update with Multiple Array Matches

The positional $ update operator behaves ambiguously when the query has multiple array fields to filter documents in the collection.

Consider a document in the students_deans_list collection, which holds arrays of student information:

db.students_deans_list.insertMany( [
   {
      _id: 8,
      activity_ids: [ 1, 2 ],
      grades: [ 90, 95 ],
      deans_list: [ 2021, 2020 ]
   }
] )

In the following example, the user attempts to modify the deans_list field. The example filters documents using the activity_ids, deans_list, and grades fields, and updates the 2021 value in the deans_list field to 2022:

db.students_deans_list.updateOne(
   { activity_ids: 1, grades: 95, deans_list: 2021 },
   { $set: { "deans_list.$": 2022 } }
)

When the server executes the preceding updateOne method, it filters the available documents using values in the supplied array fields. Although the deans_list field is used in the filter, it is not the field used by the positional $ update operator to determine which position in the array to update:

db.students_deans_list.find( { _id: 8 } )

Example output:

{
   _id: 8,
   activity_ids: [ 1, 2 ],
   grades: [ 90, 95 ],
   deans_list: [ 2021, 2022 ]
}

The updateOne method matched the deans_list field on 2021, but the positional $ update operator instead changed the 2020 value to 2022.

To avoid unexpected results when matching on multiple arrays, use the filtered positional operator $[<identifier>].