$addToSet
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Definition
$addToSetThe
$addToSetoperator adds a value to an array unless the value is already present, in which case$addToSetdoes nothing to that array.The
$addToSetoperator has the form:{ $addToSet: { <field1>: <value1>, ... } } To specify a
<field>in an embedded document or in an array, use dot notation.
Behavior
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.
$addToSet only ensures that there are no duplicate items
added to the set and does not affect existing duplicate elements.
$addToSet does not guarantee a particular ordering of
elements in the modified set.
Starting in MongoDB 5.0, mongod no longer raises an
error when you use an update operator like $addToSet
with an empty operand expression ( { } ). An empty update results
in no changes and no oplog entry is created (meaning that the
operation is a no-op).
Missing Field
If you use $addToSet on a field that is absent from the
document to update, $addToSet creates the array field with
the specified value as its element.
Field is Not an Array
If you use $addToSet on a field that is not an array, the
operation will fail.
For example, create the pigments collection:
db.pigments.insertOne( { _id: 1, colors: "blue, green, red" } )
The colors field is not an array. The following $addToSet
operation fails:
db.pigments.updateOne( { _id: 1 }, { $addToSet: { colors: "mauve" } } )
Value to Add is An Array
If the value is an array, $addToSet appends the whole array
as a single element.
Create the alphabet collection:
db.alphabet.insertOne( { _id: 1, letters: ["a", "b"] } )
The following operation appends the array [ "c", "d" ] to the
letters field:
db.alphabet.updateOne( { _id: 1 }, { $addToSet: { letters: [ "c", "d" ] } } )
The array [ "c", "d" ] is added as a single element:
{ _id: 1, letters: [ 'a', 'b', [ 'c', 'd' ] ] }
Tip
To add each element of the value separately, use the
$each modifier with $addToSet. See
$each Modifier for details.
Value to Add is a Document
If the value is a document, MongoDB determines that the document is a duplicate if an existing document in the array matches the to-be-added document exactly; i.e. the existing document has the exact same fields and values and the fields are in the same order. As such, field order matters and you cannot specify that MongoDB compare only a subset of the fields in the document to determine whether the document is a duplicate of an existing array element.
Examples
Create the inventory collection:
db.inventory.insertOne( { _id: 1, item: "polarizing_filter", tags: [ "electronics", "camera" ] } )
Add to Array
The following operation adds the element "accessories" to the
tags array since "accessories" does not exist in the array:
db.inventory.updateOne( { _id: 1 }, { $addToSet: { tags: "accessories" } } )
Value Already Exists
The following $addToSet operation has no effect because
"camera" is already an element of the tags array:
db.inventory.updateOne( { _id: 1 }, { $addToSet: { tags: "camera" } } )
$each Modifier
You can use the $addToSet operator with the
$each modifier. The $each modifier allows the
$addToSet operator to add multiple values to the array
field.
A collection inventory has the following document:
{ _id: 2, item: "cable", tags: [ "electronics", "supplies" ] }
Then the following operation uses the $addToSet operator
with the $each modifier to add multiple elements to the
tags array:
db.inventory.updateOne( { _id: 2 }, { $addToSet: { tags: { $each: [ "camera", "electronics", "accessories" ] } } } )
The operation only adds "camera" and "accessories" to the
tags array. "electronics" was already in the array:
{ _id: 2, item: "cable", tags: [ "electronics", "supplies", "camera", "accessories" ] }