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Aggregation Pipeline Operators

$filter (aggregation)

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  • Definition
  • Compatibility
  • Syntax
  • Behavior
  • Example

Definition

$filter

New in version 3.2.

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.

Compatibility

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

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

  • MongoDB Enterprise: The subscription-based, self-managed version of MongoDB

  • MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB

Syntax

$filter has the following syntax:

{
    $filter:
      {
         input: <array>,
         cond: <expression>,
         as: <string>,
         limit: <number expression>
      }
}
Field
Specification

input

An expression that resolves to an array.

If input resolves to null or refers to a missing field, $filter returns null.

If input resolves to a non-array, non-null value, the pipeline errors.

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.

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.

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 Expressions.

Behavior

Example
Results
{
  $filter: {
     input: [ 1, "a", 2, null, 3.1, NumberLong(4), "5" ],
     as: "num",
     cond: { $and: [
        { $gte: [ "$$num", NumberLong("-9223372036854775807") ] },
        { $lte: [ "$$num", NumberLong("9223372036854775807") ] }
      ] }
  }
}

[ 1, 2, 3.1, NumberLong(4) ]

Example

A collection sales has the following documents:

{
   _id: 0,
   items: [
     { item_id: 43, quantity: 2, price: 10 },
     { item_id: 2, quantity: 1, price: 240 }
   ]
}
{
   _id: 1,
   items: [
     { item_id: 23, quantity: 3, price: 110 },
     { item_id: 103, quantity: 4, price: 5 },
     { item_id: 38, quantity: 1, price: 300 }
   ]
}
{
    _id: 2,
    items: [
       { item_id: 4, quantity: 1, price: 23 }
    ]
}

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 ] }
            }
         }
      }
   }
])

The operation produces the following results:

{
   "_id" : 0,
   "items" : [
      { "item_id" : 2, "quantity" : 1, "price" : 240 }
   ]
}
{
   "_id" : 1,
   "items" : [
      { "item_id" : 23, "quantity" : 3, "price" : 110 },
      { "item_id" : 38, "quantity" : 1, "price" : 300 }
   ]
}
{ "_id" : 2, "items" : [ ] }

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