How to Use $filter in Aggregation in MongoDB

In MongoDB aggregation, use the $filter operator to select elements from an array that meet a specified condition. It takes an input array, a condition expression, and returns a filtered array with only matching elements.

Syntax

The $filter operator has three main parts:

• input: The array to filter.
• as: A variable name for each element in the array.
• cond: A condition expression that returns true for elements to keep.

json

{
  $filter: {
    input: <array>,
    as: <variable_name>,
    cond: <condition_expression>
  }
}

Example

This example filters an array of scores to keep only those greater than 70.

mongodb

db.students.aggregate([
  {
    $project: {
      name: 1,
      highScores: {
        $filter: {
          input: "$scores",
          as: "score",
          cond: { $gt: ["$$score", 70] }
        }
      }
    }
  }
])

Output

[
  { "_id": ObjectId("..."), "name": "Alice", "highScores": [85, 90] },
  { "_id": ObjectId("..."), "name": "Bob", "highScores": [75] }
]

Common Pitfalls

Common mistakes when using $filter include:

• Not using $$ before the variable name inside cond. For example, use $$score not $score.
• Passing a non-array value to input, which causes errors.
• Using incorrect condition syntax that does not return a boolean.

json

/* Wrong: missing $$ before variable */
{
  $filter: {
    input: "$scores",
    as: "score",
    cond: { $gt: ["$score", 70] }  // Incorrect
  }
}

/* Correct: use $$score */
{
  $filter: {
    input: "$scores",
    as: "score",
    cond: { $gt: ["$$score", 70] }  // Correct
  }
}

Quick Reference

Part	Description	Example
input	Array to filter	"$scores"
as	Variable name for each element	"score"
cond	Condition to keep element	{ $gt: ["$$score", 70] }

Key Takeaways

Use $filter in aggregation to select array elements matching a condition.

Always prefix the variable in cond with $$ to reference array elements.

Input must be an array; otherwise, $filter will error.

The cond expression must return true or false for each element.

Use $filter inside $project or other pipeline stages to reshape documents.