$filter (aggregation)¶

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Definition
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.

$filter has the following syntax:

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{ $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

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{
  $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:

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{
   _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:

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db.sales.aggregate([
   {
      $project: {
         items: {
            $filter: {
               input: "$items",
               as: "item",
               cond: { $gte: [ "$$item.price", 100 ] }
            }
         }
      }
   }
])

The operation produces the following results:

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{
   "_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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