Chapter•60 Minutes

Items per page

Cheat Sheet: Utility Operators, Variables, and BSON Types

Utility expressions, system variables, BSON type reminders, missing/null semantics, and collation.

$literal

Reference type:Cheat sheetFormat:Syntax + example + use case

Reference group: Utility expressions and variables

What it does

Treats a value literally rather than interpreting a leading $ as a field path.

Syntax

{$literal: value}

Use case

Output strings that resemble field references.

$literal: practical example

{ $project: { literalText: { $literal: "$notAField" } } }

💡 Explanation Treats a value literally rather than interpreting a leading $ as a field path. When to use it Output strings that resemble field references.

Key Takeaways

Treats a value literally rather than interpreting a leading $ as a field path.
Typical use: Output strings that resemble field references.

$let

Reference type:Cheat sheetFormat:Syntax + example + use case

Reference group: Utility expressions and variables

What it does

Defines scoped variables referenced with $$name.

Syntax

{$let: { vars: { name: expression }, in: expression }}

Use case

Avoid repeating complex expressions.

$let: practical example

{ $project: { total: { $let: { vars: { subtotal: { $multiply: ["$price", "$quantity"] } }, in: { $add: ["$$subtotal", "$shipping"] } } } }

💡 Explanation Defines scoped variables referenced with $$name. When to use it Avoid repeating complex expressions.

Key Takeaways

Defines scoped variables referenced with $$name.
Typical use: Avoid repeating complex expressions.

$$ROOT

Reference type:Cheat sheetFormat:Syntax + example + use case

Reference group: Utility expressions and variables

What it does

References the full current pipeline document.

Syntax

$$ROOT

Use case

Preserve or collect complete documents.

$$ROOT: practical example

{ $replaceWith: "$$ROOT" }

💡 Explanation References the full current pipeline document. When to use it Preserve or collect complete documents.

Key Takeaways

References the full current pipeline document.
Typical use: Preserve or collect complete documents.

$$CURRENT

Reference type:Cheat sheetFormat:Syntax + example + use case

Reference group: Utility expressions and variables

What it does

References current document context; it often starts equal to $$ROOT but can change in scoped expressions.

Syntax

$$CURRENT

Use case

Understand expression scope.

$$CURRENT: practical example

{ $project: { currentId: "$$CURRENT._id" } }

💡 Explanation References current document context; it often starts equal to $$ROOT but can change in scoped expressions. When to use it Understand expression scope.

Key Takeaways

References current document context; it often starts equal to $$ROOT but can change in scoped expressions.
Typical use: Understand expression scope.

$$REMOVE

Reference type:Cheat sheetFormat:Syntax + example + use case

Reference group: Utility expressions and variables

What it does

Removes a field in supported projection/set contexts.

Syntax

$$REMOVE

Use case

Conditionally omit fields instead of outputting null.

$$REMOVE: practical example

{ $project: { optional: { $cond: [{ $eq: ["$include", true] }, "$value", "$$REMOVE"] } } }

💡 Explanation Removes a field in supported projection/set contexts. When to use it Conditionally omit fields instead of outputting null.

Key Takeaways

Removes a field in supported projection/set contexts.
Typical use: Conditionally omit fields instead of outputting null.

$$NOW

Reference type:Cheat sheetFormat:Syntax + example + use case

Reference group: Utility expressions and variables

What it does

Current datetime value consistent throughout a pipeline.

Syntax

$$NOW

Use case

Timestamp computed output.

$$NOW: practical example

{ $set: { calculatedAt: "$$NOW" } }

💡 Explanation Current datetime value consistent throughout a pipeline. When to use it Timestamp computed output.

Key Takeaways

Current datetime value consistent throughout a pipeline.
Typical use: Timestamp computed output.

$type/$isNumber/$isArray

Reference type:Cheat sheetFormat:Syntax + example + use case

Reference group: Utility expressions and variables

What it does

Inspects BSON types before applying type-specific operations.

Syntax

Type inspection expressions

Use case

Audit heterogeneous data.

$type/$isNumber/$isArray: practical example

{ $project: { kind: { $type: "$payload" }, isArray: { $isArray: "$items" }, numeric: { $isNumber: "$price" } } }

💡 Explanation Inspects BSON types before applying type-specific operations. When to use it Audit heterogeneous data.

Key Takeaways

Inspects BSON types before applying type-specific operations.
Typical use: Audit heterogeneous data.

$mergeObjects

Reference type:Cheat sheetFormat:Syntax + example + use case

Reference group: Utility expressions and variables

What it does

Merges objects; later non-null objects override duplicate keys.

Syntax

{$mergeObjects: [object1, object2, ...]}

Use case

Apply defaults and overrides.

$mergeObjects: practical example

{ $project: { settings: { $mergeObjects: [{ theme: "light", compact: false }, "$preferences"] } }

💡 Explanation Merges objects; later non-null objects override duplicate keys. When to use it Apply defaults and overrides.

Key Takeaways

Merges objects; later non-null objects override duplicate keys.
Typical use: Apply defaults and overrides.

$getField/$setField

Reference type:Cheat sheetFormat:Syntax + example + use case

Reference group: Utility expressions and variables

What it does

Accesses or updates object fields, including unusual field names in supported versions.

Syntax

Object field access/update expressions

Use case

Handle dynamic keys.

$getField/$setField: practical example

{ $project: { special: { $getField: { field: "price.usd", input: "$metrics" } } } }

💡 Explanation Accesses or updates object fields, including unusual field names in supported versions. When to use it Handle dynamic keys.

Key Takeaways

Accesses or updates object fields, including unusual field names in supported versions.
Typical use: Handle dynamic keys.

ObjectId

Reference type:Cheat sheetFormat:Syntax + example + use case

Reference group: BSON types and field properties

What it does

A 12-byte identifier commonly used for _id; includes a timestamp component.

Syntax

ObjectId("24-hex-character-string")

Use case

Document identifiers and references.

Important notes

Use explicit business timestamps for business events.

ObjectId: practical example

ObjectId("65a000000000000000000001")

💡 Explanation A 12-byte identifier commonly used for _id; includes a timestamp component. When to use it Document identifiers and references. Caution Use explicit business timestamps for business events.

Key Takeaways

A 12-byte identifier commonly used for _id; includes a timestamp component.
Typical use: Document identifiers and references.

$literal

$let

$$ROOT

$$CURRENT

$$REMOVE

$$NOW

$type/$isNumber/$isArray

$mergeObjects

$getField/$setField

ObjectId

STAY UPDATED WITH BYTES!