$arrayToObject (Convert Array of Pairs to Object)

Last updated: 2026-10-09

$arrayToObject (Convert Array of Pairs to Object)

$arrayToObject converts an array of key-value pairs back into a document, the inverse of $objectToArray. The input accepts two shapes: an array of { k, v } objects, or an array of two-element arrays [ 'fieldName', value ]. Typical uses include reassembling dynamic fields after $objectToArray processing or packing grouped stats into an object. Keys must be strings; duplicate keys are overwritten by the later value.

📝 Syntax

{ $arrayToObject: <array> }

⚙️ Parameters

Returns：The resulting document; null for an empty array or one containing null

▶ Example

Run the commands below in your local mongosh (requires a running mongod):

// $arrayToObject: convert a k/v array back to an object
db.users.aggregate([
  { $project: {
      scoreObj: { $arrayToObject: '$scorePairs' }
  } }
])

Output：

{ '_id': 1, 'scoreObj': { 'math': 90, 'eng': 85 } }

📌 Notes

• Keys must be strings; a duplicate key is overwritten by the later value
• An empty array or one containing null yields null
• The { k, v } and [ 'k', v ] shapes are equivalent and may be mixed

🏷️ Related Tags

💡 Related Cases

📚 Related Tutorials

❓ FAQ

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