upsert

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

Nested Arrays

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

Unsets

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

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.

Update Values in an Array

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:

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 ] }

Update Documents in an Array

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:

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

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 }
  ]
}