08 - Expressions Overview

Expressions Overview

The language inside stages — how to compute, compare, and transform values.

← Previous | Index | Next: 09 - Arithmetic Operators →

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What Are Expressions?

Expressions are formulas used within pipeline stages to process document data. They let you compute new values, compare fields, manipulate arrays, and control logic.

Key rules:

• An expression always operates on the current document — it cannot see other documents (except in $group stages).
• Certain expressions can only be used with certain stage operators.
• The expression syntax is recursive — you can nest expressions to build complex computations.

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Expression Syntax

An expression can be one of four things:

<expression> = <field path>           // reference a field: "$fieldName"
             | <literal value>        // a constant: 42, "hello", true
             | <system variable>      // $$CURRENT, $$ROOT, etc.
             | <expression operator>  // { $add: [...] }, { $cond: {...} }

An expression operator takes one or more expressions as arguments:

// Single-argument form
{ <operator>: <expression> }
 
// Multi-argument form
{ <operator>: [<expr1>, <expr2>, ...] }

Nesting expressions

Because operators take expressions as arguments, you can nest them freely:

{
  $multiply: [
    { $add: [3, 3] },     // inner: 3 + 3 = 6
    10                      // 6 × 10 = 60
  ]
}

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Field Paths: The $ Operator

The $ prefix accesses a field’s value in the current document:

"$title"           // → value of the "title" field
"$fundings.amount" // → value of nested field "amount" in "fundings"
"$reviews"         // → the entire reviews array

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Categories of Expression Operators

Category	Operators	Description
Arithmetic	$add, $subtract, $multiply, $divide, $abs	Math on numbers
Comparison	$eq, $ne, $gt, $gte, $lt, $lte, $cmp	Compare two values
Boolean	$and, $or, $not	Combine conditions
Control Flow	$cond, $switch	If/else, switch/case
Array	$arrayElemAt, $map, $filter, $reduce, $in, $concatArrays, $isArray	Process arrays
Accumulators	$sum, $avg, $min, $max, $push, $addToSet	Aggregate across documents
Variables	$$CURRENT, $expr, $let	Reference variables
Set	$setDifference, $setIntersection, $setUnion, $setIsSubset	Set operations on arrays

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Where Expressions Are Used

Expressions appear inside pipeline stages:

db.projects.aggregate([
  {
    $addFields: {                          // stage operator
      totalFunding: { $sum: "$fundings.amount" },  // expression
      isApproved: { $eq: ["$state", "APPROVED"] }  // expression
    }
  }
]);

The stage ($addFields) determines where expressions are evaluated. The expressions themselves determine what is computed.

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Next: 09 - Arithmetic Operators — start with the simplest expressions: math.