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- $ (update)
$ (update)¶
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Definition¶
- $¶
The positional $ operator identifies an element in an array 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.
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 } } )
Behavior¶
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.
Examples¶
Update Values in an Array¶
Consider a collection students with the following documents:
{ "_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:
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.
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: 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:
db.students.update(
{ _id: 4, "grades.grade": 85 },
{ $set: { "grades.$.std" : 6 } }
)
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: 4,
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.update(
{
_id: 4,
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: 4,
grades: [
{ grade: 80, mean: 75, std: 8 },
{ grade: 85, mean: 90, std: 6 },
{ grade: 90, mean: 85, std: 3 }
]
}
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