The $cond operator adds if-then-else logic inside MongoDB aggregation pipelines. Use it to label records, compute derived fields, or return different values based on a condition.
Branch on true/false.
if, then, else keys.
Pair with $gte, $eq.
Add computed fields.
Grades, status, defaults.
Multi-tier decisions.
In MongoDB’s aggregation framework, the $cond operator evaluates a boolean condition and returns one of two expressions. Think of it as an inline if-then-else statement inside your pipeline. For example, assign "Pass" when a score is at least 70, otherwise "Fail".
The if branch must evaluate to a boolean (true or false). Use comparison operators like $gte, $eq, or $and to build your condition.
$cond supports two equivalent forms. Beginners often find the object form easiest to read:
{ $cond: { if: <condition>, then: <expression>, else: <expression> } }Shorthand array form (same behavior):
{ $cond: [ <condition>, <then-expression>, <else-expression> ] }if — a boolean expression (often built with $gte, $lt, $eq, $and, etc.).then — returned when the condition is true.else — returned when the condition is false.then and else can be strings, numbers, field references, or nested operators.$project, $addFields, $set, or $group.$cond inside else (or then) for multi-way branching.$cond for comparisons; use $ifNull for simple null fallbacks| Question | Answer |
|---|---|
| Operator type | Aggregation expression operator (conditional) |
| Object syntax | { $cond: { if, then, else } } |
| Array syntax | { $cond: [ condition, then, else ] } |
| Condition type | Must evaluate to boolean |
| Common stages | $project, $addFields, $set, $group |
{
$cond: {
if: {
$gte: [
"$score",
70
]
},
then: "Pass",
else: "Fail"
}
}Grade by threshold
{
$cond: [
{
$eq: [
"$status",
"active"
]
},
1,
0
]
}Shorthand syntax
{
$cond: [
{
$gt: [
"$qty",
0
]
},
"$qty",
0
]
}Replace invalid qty
else: {
$cond: [
...
]
}Multi-tier logic
Label student grades, set inventory status, apply defaults, and build multi-tier letter grades with $cond.
Use a students collection and assign Pass or Fail based on score.
Suppose you have a students collection with name and score fields:
[
{ "_id": ObjectId("609c26812e9274a86871bc6a"), "name": "Alice", "score": 85 },
{ "_id": ObjectId("609c26812e9274a86871bc6b"), "name": "Bob", "score": 70 },
{ "_id": ObjectId("609c26812e9274a86871bc6c"), "name": "Charlie", "score": 60 }
]Assign "Pass" when score >= 70, otherwise "Fail":
db.students.aggregate([
{
$project: {
name: 1,
score: 1,
grade: {
$cond: {
if: { $gte: ["$score", 70] },
then: "Pass",
else: "Fail"
}
}
}
}
])$project keeps name and adds a computed grade field.$gte: ["$score", 70] returns true when score is 70 or higher.Inventory labels, numeric defaults, and nested multi-tier grading.
Label products as "In Stock" or "Out of Stock" based on quantity:
db.products.aggregate([
{
$addFields: {
stockStatus: {
$cond: [
{ $gt: ["$quantity", 0] },
"In Stock",
"Out of Stock"
]
}
}
}
])
// quantity: 5 → "In Stock"
// quantity: 0 → "Out of Stock"This example uses the array shorthand form of $cond. $addFields adds the new field while keeping all existing fields in each document.
Return the quantity when positive, otherwise use 0:
db.orders.aggregate([
{
$project: {
product: 1,
safeQty: {
$cond: {
if: { $gt: ["$quantity", 0] },
then: "$quantity",
else: 0
}
}
}
}
])The then branch can reference a field ("$quantity") instead of a literal. This pattern sanitizes bad or negative values before reporting.
Chain conditions to assign A, B, C, or F:
db.students.aggregate([
{
$project: {
name: 1,
score: 1,
letterGrade: {
$cond: {
if: { $gte: ["$score", 90] },
then: "A",
else: {
$cond: {
if: { $gte: ["$score", 80] },
then: "B",
else: {
$cond: {
if: { $gte: ["$score", 70] },
then: "C",
else: "F"
}
}
}
}
}
}
}
}
])
// score 85 → "B"
// score 72 → "C"
// score 60 → "F"Each else branch contains another $cond, creating a decision tree. For many branches, consider $switch as a cleaner alternative.
$cond WorksComparison or logical operators resolve to true or false for each document.
True → then expression. False → else expression.
The chosen value becomes the new field value in $project or $addFields.
Each document gets its own result based on its data — no application code required.
The $cond operator brings if-then-else logic directly into MongoDB aggregation pipelines. It is ideal for labeling, grading, status fields, and conditional transformations without post-processing in your application.
Start with simple pass/fail conditions, then explore nested $cond or $switch for multi-way branching. Pair with comparison operators like $gte and $eq to build clear, readable conditions.
$gte, $eq, $and, $orthen and else the same type when possible$switch when you have many branchesif branch$cond operators without considering $switch$cond with $ifNull for general comparisons$gte uses an array: ["$score", 70]$condUse these points when adding conditional logic in MongoDB.
Pipeline branching.
PurposeObject or array form.
SyntaxUse $gte, $eq, etc.
Rulethen/else can nest ops.
FlexibilityGeneral conditions.
ImportantMove on to $convert for type conversion, or review $concatArrays for merging arrays.
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