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Cheat Sheet: Array Operators

Array query predicates, aggregation expressions, update modifiers, and positional update selectors.

$all

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

Reference group: Array query operators

What it does

Requires an array to contain every listed value.

Syntax

{ field: { $all: [v1, v2] }}

Use case

Find records with all required labels.

$all: practical example

db.products.find({ tags: { $all: ["mongodb", "database"] } })

💡 Explanation Requires an array to contain every listed value. When to use it Find records with all required labels.

Key Takeaways

Requires an array to contain every listed value.
Typical use: Find records with all required labels.

$elemMatch

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

Reference group: Array query operators

What it does

Requires one array element to satisfy all inner conditions.

Syntax

{ array: { $elemMatch: { ... } } }

Use case

Match multiple predicates on the same embedded item.

$elemMatch: practical example

db.orders.find({ items: { $elemMatch: { sku: "KB-1", quantity: { $gte: 2 } } } })

💡 Explanation Requires one array element to satisfy all inner conditions. When to use it Match multiple predicates on the same embedded item.

Key Takeaways

Requires one array element to satisfy all inner conditions.
Typical use: Match multiple predicates on the same embedded item.

$size

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

Reference group: Array query operators

What it does

Matches arrays with exactly n elements.

Syntax

{ arrayField: { $size: n } }

Use case

Find arrays of exact length.

Important notes

Query $size does not directly support arbitrary ranges.

$size: practical example

db.posts.find({ tags: { $size: 3 } })

💡 Explanation Matches arrays with exactly n elements. When to use it Find arrays of exact length. Caution Query $size does not directly support arbitrary ranges.

Key Takeaways

Matches arrays with exactly n elements.
Typical use: Find arrays of exact length.

$in

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

Reference group: Array query operators

What it does

Matches a value equal to any listed item; for arrays, a matching element is sufficient.

Syntax

{ field: { $in: [values] } }

Use case

Filter by allowed categories.

$in: practical example

db.products.find({ category: { $in: ["keyboard", "mouse"] } })

💡 Explanation Matches a value equal to any listed item; for arrays, a matching element is sufficient. When to use it Filter by allowed categories.

Key Takeaways

Matches a value equal to any listed item; for arrays, a matching element is sufficient.
Typical use: Filter by allowed categories.

$nin

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

Reference group: Array query operators

What it does

Matches values not in the list and also matches missing fields.

Syntax

{ field: { $nin: [values] } }

Use case

Exclude statuses.

Important notes

Use $exists if missing fields must not match.

$nin: practical example

db.products.find({ status: { $nin: ["deleted", "blocked"] } })

💡 Explanation Matches values not in the list and also matches missing fields. When to use it Exclude statuses. Caution Use $exists if missing fields must not match.

Key Takeaways

Matches values not in the list and also matches missing fields.
Typical use: Exclude statuses.

$isArray

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

Reference group: Array aggregation expressions

What it does

Returns true if the expression resolves to an array.

Syntax

{$isArray: expression}

Use case

Guard against inconsistent imported data.

$isArray: practical example

{ $project: { isArray: { $isArray: "$tags" } } }

💡 Explanation Returns true if the expression resolves to an array. When to use it Guard against inconsistent imported data.

Key Takeaways

Returns true if the expression resolves to an array.
Typical use: Guard against inconsistent imported data.

$size

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

Reference group: Array aggregation expressions

What it does

Returns array length; errors if the resolved value is not an array.

Syntax

{$size: arrayExpr}

Use case

Calculate array length.

Important notes

Use $ifNull and validation for missing/null values.

$size: practical example

{ $project: { tagCount: { $size: { $ifNull: ["$tags", []] } } } }

💡 Explanation Returns array length; errors if the resolved value is not an array. When to use it Calculate array length. Caution Use $ifNull and validation for missing/null values.

Key Takeaways

Returns array length; errors if the resolved value is not an array.
Typical use: Calculate array length.

$arrayElemAt

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

Reference group: Array aggregation expressions

What it does

Returns the element at an index; negative indexes count from the end.

Syntax

{$arrayElemAt: [array, index]}

Use case

Select a boundary element from an ordered array.

$arrayElemAt: practical example

{ $project: { latest: { $arrayElemAt: ["$events", -1] } } }

💡 Explanation Returns the element at an index; negative indexes count from the end. When to use it Select a boundary element from an ordered array.

Key Takeaways

Returns the element at an index; negative indexes count from the end.
Typical use: Select a boundary element from an ordered array.

$filter

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

Reference group: Array aggregation expressions

What it does

Keeps elements satisfying a condition and preserves relative order.

Syntax

{$filter: { input, as?, cond, limit? }}

Use case

Filter arrays without unwinding.

$filter: practical example

{ $project: { passing: { $filter: { input: "$scores", as: "s", cond: { $gte: ["$$s", 70] } } } } }

💡 Explanation Keeps elements satisfying a condition and preserves relative order. When to use it Filter arrays without unwinding.

Key Takeaways

Keeps elements satisfying a condition and preserves relative order.
Typical use: Filter arrays without unwinding.

$map

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

Reference group: Array aggregation expressions

What it does

Transforms each element into a new value or object.

Syntax

{$map: { input, as?, in }}

Use case

Compute per-element values.

$map: practical example

{ $project: { lines: { $map: { input: "$items", as: "i", in: { sku: "$$i.sku", total: { $multiply: ["$$i.price", "$$i.qty"] } } } } } }

💡 Explanation Transforms each element into a new value or object. When to use it Compute per-element values.

Key Takeaways

Transforms each element into a new value or object.
Typical use: Compute per-element values.

$all

$elemMatch

$size

$in

$nin

$isArray

$size

$arrayElemAt

$filter

$map

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