$[]

upsert

If an upsert operation results in an insert, the query must include an exact equality match on the array field in order to use the $[] positional operator in the update statement.

For example, the following upsert operation, which uses $[] in the update document, specifies an exact equality match condition on the array field:

db.collection.update(
   { myArray: [ 5, 8 ] },
   { $set: { "myArray.$[]": 10 } },
   { upsert: true }
)

If no such document exists, the operation would result in an insertion of the following document:

{ "_id" : ObjectId(...), "myArray" : [ 10, 10 ] }

If the upsert operation did not include an exact equality match and no matching documents were found to update, the upsert operation would error.

For example, the following operations would error if no matching documents were found to update:

db.collection.update(
   { myArray: 5 },
   { $set: { "myArray.$[]": 10 } },
   { upsert: true }
)
db.collection.update(
   { },
   { $set: { "myArray.$[]": 10 } },
   { upsert: true }
)

Nested Arrays

The $[] operator can be used for queries which traverse more than one array and nested arrays.

For an example, see Update Nested Arrays in Conjunction with $[<identifier>].

Update All Elements in an Array

Consider a collection students with the following documents:

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

To increment all elements in the grades array by 10 for all documents in the collection, use the all positional $[] operator:

db.students.update(
   { },
   { $inc: { "grades.$[]": 10 } },
   { multi: true }
)

The all positional $[] operator acts as a placeholder for all elements in the array field.

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

{ "_id" : 1, "grades" : [ 95, 92, 90 ] }
{ "_id" : 2, "grades" : [ 98, 100, 102 ] }
{ "_id" : 3, "grades" : [ 95, 110, 100 ] }

Update All Documents in an Array

The $[] positional operator facilitates updates to arrays that contain embedded documents. To access the fields in the embedded documents, use the dot notation on the $[] operator.

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

Consider a collection students2 with the following documents:

{
   "_id" : 1,
   "grades" : [
      { "grade" : 80, "mean" : 75, "std" : 8 },
      { "grade" : 85, "mean" : 90, "std" : 6 },
      { "grade" : 85, "mean" : 85, "std" : 8 }
   ]
}
{
   "_id" : 2,
   "grades" : [
      { "grade" : 90, "mean" : 75, "std" : 8 },
      { "grade" : 87, "mean" : 90, "std" : 5 },
      { "grade" : 85, "mean" : 85, "std" : 6 }
   ]
}

To modify the value of the std field for all elements in the grades array, use the positional $[] operator:

db.students2.update(
   { },
   { $inc: { "grades.$[].std" : -2 } },
   { multi: true }
)

After the operation, the collection has the following documents:

{
   "_id" : 1,
   "grades" : [
      { "grade" : 80, "mean" : 75, "std" : 6 },
      { "grade" : 85, "mean" : 90, "std" : 4 },
      { "grade" : 85, "mean" : 85, "std" : 6 }
   ]
}
{
   "_id" : 2,
   "grades" : [
      { "grade" : 90, "mean" : 75, "std" : 6 },
      { "grade" : 87, "mean" : 90, "std" : 3 },
      { "grade" : 85, "mean" : 85, "std" : 4 }
   ]
}

Update Arrays Specified Using a Negation Query Operator

Consider a collection results with the following documents:

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

To increment all elements in the grades array by 10 for all documents except those with the value 100 in the grades array, use the all positional $[] operator:

db.results.update(
   { "grades" : { $ne: 100 } },
   { $inc: { "grades.$[]": 10 } },
   { multi: true }
)

The all positional $[] operator acts as a placeholder for all elements in the array field.

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

{ "_id" : 1, "grades" : [ 95, 92, 90 ] }
{ "_id" : 2, "grades" : [ 98, 100, 102 ] }
{ "_id" : 3, "grades" : [ 85, 100, 90 ] }

Update Nested Arrays in Conjunction with $[<identifier>]

The $[] positional operator, in conjunction with filter $[<identifier>] positional operator can be used to update nested arrays.

Create a collection students3 with the following documents:

db.students3.insert([
   { "_id" : 1,
      "grades" : [
        { type: "quiz", questions: [ 10, 8, 5 ] },
        { type: "quiz", questions: [ 8, 9, 6 ] },
        { type: "hw", questions: [ 5, 4, 3 ] },
        { type: "exam", questions: [ 25, 10, 23, 0 ] },
      ]
   }
])

To update all values that are greater than or equal to 8 in the nested grades.questions array, regardless of type:

db.students3.update(
   {},
   { $inc: { "grades.$[].questions.$[score]": 2 } },
   { arrayFilters: [  { "score": { $gte: 8 } } ], multi: true}
)