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$filter (aggregation)

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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.

$filter has the following syntax:

{ $filter: { input: <array>, as: <string>, cond: <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.

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" : [ ] }