How to Use $ifNull in Aggregation in MongoDB

In MongoDB aggregation, use $ifNull to replace null or missing values with a specified default. It takes exactly two arguments: the expression to check and the replacement value if the expression is null or missing.

Syntax

The $ifNull operator takes exactly two arguments inside an array. The first is the expression to check for null or missing values. The second is the value to use if the first is null or missing.

Syntax pattern:

{ $ifNull: [ <expression>, <replacement> ] }

json

{ $ifNull: [ <expression>, <replacement> ] }

Example

This example shows how to use $ifNull in an aggregation pipeline to replace null or missing score fields with a default value of 0.

mongodb

db.students.aggregate([
  {
    $project: {
      name: 1,
      score: { $ifNull: ["$score", 0] }
    }
  }
])

Output

[
  { "_id" : ObjectId("..."), "name" : "Alice", "score" : 85 },
  { "_id" : ObjectId("..."), "name" : "Bob", "score" : 0 },
  { "_id" : ObjectId("..."), "name" : "Charlie", "score" : 92 }
]

Common Pitfalls

• Using $ifNull with only one argument causes an error; it requires exactly two.
• It only checks for null or missing fields, not other falsy values like 0 or "".
• Confusing $ifNull with $coalesce which handles multiple expressions.

mongodb

/* Wrong usage: missing second argument */
db.collection.aggregate([
  { $project: { value: { $ifNull: ["$field"] } } }
])

/* Correct usage: provide replacement value */
db.collection.aggregate([
  { $project: { value: { $ifNull: ["$field", "default"] } } }
])

Quick Reference

$ifNull checks if the first expression is null or missing and returns the second expression if true, otherwise returns the first.

• Arguments: exactly two
• Returns: first expression if not null, else second
• Use case: default values in aggregation

Feature	Description
Arguments	Two: expression to check, replacement value
Returns	First expression if not null, else replacement
Use Case	Provide default values for null or missing fields
Error	Throws if less or more than two arguments

Key Takeaways

Use $ifNull with exactly two arguments: the value to check and the default replacement.

$ifNull returns the replacement only if the first value is null or missing, not for other falsy values.

Common mistake is to omit the replacement argument, which causes errors.

$ifNull is useful in aggregation pipelines to ensure fields have fallback values.

Remember $ifNull differs from $coalesce, which accepts multiple expressions.