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$filter (aggregation)

On this page

  • Definition
  • Compatibility
  • Syntax
  • Behavior
  • Examples
  • Filter Based on Number Comparison
  • Use the limit Field
  • limit Greater than Possible Matches
  • Filter Based on String Equality Match
  • Filter Based on Regular Expression Match
$filter

Selects a subset of an array to return based on the specified condition. Returns an array with only those elements that match the condition. The returned elements are in the original order.

You can use $filter for deployments hosted in the following environments:

  • MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

$filter has the following syntax:

{
$filter:
{
input: <array>,
as: <string>,
cond: <expression>,
limit: <number expression>
}
}
Field
Specification
input
An expression that resolves to an array.
as
Optional. A name for the variable that represents each individual element of the input array. If no name is specified, the variable name defaults to this.
cond
An expression that resolves to a boolean value used to determine if an element should be included in the output array. The expression references each element of the input array individually with the variable name specified in as.
limit

Optional. A number expression that restricts the number of matching array elements that $filter returns. You cannot specify a limit less than 1. The matching array elements are returned in the order they appear in the input array.

If the specified limit is greater than the number of matching array elements, $filter returns all matching array elements. If the limit is null, $filter returns all matching array elements.

For more information on expressions, see Expression Operators.

Example
Results
{
$filter: {
input: [ 1, "a", 2, null, 3.1, NumberLong(4), "5" ],
as: "num",
cond: { $isNumber: "$$num" }
}
}
[ 1, 2, 3.1, NumberLong(4) ]
{
$filter: {
input: [ 1, "a", 2, null, 3.1, NumberLong(4), "5" ],
as: "num",
cond: { $isNumber: "$$num" },
limit: 2
}
}
[ 1, 2 ]
{
$filter: {
input: [ 1, "a", 2, null, 3.1, NumberLong(4), "5" ],
as: "num",
cond: { $isNumber: "$$num" },
limit: { $add: [ 0, 1 ] }
}
}
[ 1 ]

A collection sales has the following documents:

db.sales.insertMany( [
{
_id: 0,
items: [
{ item_id: 43, quantity: 2, price: 10, name: "pen" },
{ item_id: 2, quantity: 1, price: 240, name: "briefcase" }
]
},
{
_id: 1,
items: [
{ item_id: 23, quantity: 3, price: 110, name: "notebook" },
{ item_id: 103, quantity: 4, price: 5, name: "pen" },
{ item_id: 38, quantity: 1, price: 300, name: "printer" }
]
},
{
_id: 2,
items: [
{ item_id: 4, quantity: 1, price: 23, name: "paper" }
]
}
] )

The following example filters the items array to only include documents that have a price greater than or equal to 100:

db.sales.aggregate( [
{
$project: {
items: {
$filter: {
input: "$items",
as: "item",
cond: { $gte: [ "$$item.price", 100 ] }
}
}
}
}
] )
[
{
_id: 0,
items: [ { item_id: 2, quantity: 1, price: 240, name: 'briefcase' } ]
},
{
_id: 1,
items: [
{ item_id: 23, quantity: 3, price: 110, name: 'notebook' },
{ item_id: 38, quantity: 1, price: 300, name: 'printer' }
]
},
{ _id: 2, items: [] }
]

This example uses the sales collection from the previous example.

The example uses the limit field to specify the number of matching elements returned in each items array.

db.sales.aggregate( [
{
$project: {
items: {
$filter: {
input: "$items",
as: "item",
cond: { $gte: [ "$$item.price", 100 ] },
limit: 1
}
}
}
}
] )
[
{
_id: 0,
items: [ { item_id: 2, quantity: 1, price: 240, name: 'briefcase' } ]
},
{
_id: 1,
items: [ { item_id: 23, quantity: 3, price: 110, name: 'notebook' } ]
},
{ _id: 2, items: [] }
]

This example uses the sales collection from the previous example.

The example uses a limit field value that is larger than the possible number of matching elements that can be returned. In this case, limit does not affect the query results and returns all documents matching the $gte filter criteria.

db.sales.aggregate( [
{
$project: {
items: {
$filter: {
input: "$items",
as: "item",
cond: { $gte: [ "$$item.price", 100] },
limit: 5
}
}
}
}
] )
[
{
_id: 0,
items: [ { item_id: 2, quantity: 1, price: 240, name: 'briefcase' } ]
},
{
_id: 1,
items: [
{ item_id: 23, quantity: 3, price: 110, name: 'notebook' },
{ item_id: 38, quantity: 1, price: 300, name: 'printer' }
]
},
{ _id: 2, items: [] }
]

This example uses the sales collection from the previous example.

The following aggregation filters for items that have a name value of pen.

db.sales.aggregate( [
{
$project: {
items: {
$filter: {
input: "$items",
as: "item",
cond: { $eq: [ "$$item.name", "pen"] }
}
}
}
}
] )
[
{
_id: 0,
items: [ { item_id: 43, quantity: 2, price: 10, name: 'pen' } ]
},
{
_id: 1,
items: [ { item_id: 103, quantity: 4, price: 5, name: 'pen' } ]
},
{ _id: 2, items: [] }
]

This example uses the sales collection from the previous example.

The following aggregation uses $regexMatch to filter for items that have a name value that starts with p:

db.sales.aggregate( [
{
$project: {
items: {
$filter: {
input: "$items",
as: "item",
cond: {
$regexMatch: { input: "$$item.name", regex: /^p/ }
}
}
}
}
}
] )
[
{
_id: 0,
items: [ { item_id: 43, quantity: 2, price: 10, name: 'pen' } ]
},
{
_id: 1,
items: [
{ item_id: 103, quantity: 4, price: 5, name: 'pen' },
{ item_id: 38, quantity: 1, price: 300, name: 'printer' }
]
},
{
_id: 2,
items: [ { item_id: 4, quantity: 1, price: 23, name: 'paper' } ]
}
]
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