How to Use $cond in Aggregation in MongoDB: Syntax and Examples

In MongoDB aggregation, use the $cond operator to apply conditional logic within a pipeline stage. It takes three parts: a condition, a value if true, and a value if false, allowing you to return different results based on the condition.

Syntax

The $cond operator has this form inside an aggregation pipeline:

• if: The condition to check (expression that returns true or false).
• then: The value to return if the condition is true.
• else: The value to return if the condition is false.

You can write it as an object with if, then, and else fields or as an array with three elements: condition, true case, false case.

json

{
  $cond: {
    if: <condition>,
    then: <value_if_true>,
    else: <value_if_false>
  }
}

// Or as an array:
{
  $cond: [ <condition>, <value_if_true>, <value_if_false> ]
}

Example

This example shows how to use $cond in an aggregation pipeline to add a new field status that says "Adult" if age is 18 or more, and "Minor" otherwise.

mongodb

db.people.aggregate([
  {
    $addFields: {
      status: {
        $cond: {
          if: { $gte: ["$age", 18] },
          then: "Adult",
          else: "Minor"
        }
      }
    }
  }
])

Output

[
  { "_id": 1, "name": "Alice", "age": 25, "status": "Adult" },
  { "_id": 2, "name": "Bob", "age": 16, "status": "Minor" },
  { "_id": 3, "name": "Charlie", "age": 18, "status": "Adult" }
]

Common Pitfalls

Common mistakes when using $cond include:

• Using incorrect syntax, like missing the if, then, or else keys when using the object form.
• Confusing the order of elements when using the array form.
• Not using expressions properly inside the condition, such as forgetting to use $gte or other operators.
• Expecting $cond to work outside aggregation pipelines.

json

/* Wrong: missing 'if' key */
{
  $cond: {
    then: "Adult",
    else: "Minor"
  }
}

/* Correct: include 'if' key */
{
  $cond: {
    if: { $gte: ["$age", 18] },
    then: "Adult",
    else: "Minor"
  }
}

Quick Reference

Part	Description	Example
if	Condition to evaluate (true/false)	{ $gte: ["$age", 18] }
then	Value if condition is true	"Adult"
else	Value if condition is false	"Minor"

Key Takeaways

Use $cond in aggregation to return values based on a condition.

You can write $cond as an object with if/then/else or as an array with three elements.

Always ensure the condition expression is valid and returns a boolean.

Common errors include missing keys or wrong order in the array form.

$cond works only inside aggregation pipeline stages like $project or $addFields.