What update operators does MongoDB provide?

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MongoDB **update operators** (prefixed with `$`) modify *specific fields* of documents in place — setting values, incrementing numbers, manipulating arrays, and more. They're essential for updating documents correctly (without replacing the whole document) and enable atomic, targeted modifications. ## Field update operators ```js db.users.updateOne({ _id: id }, { $set: { name: "Ann", "address.city": "LA" }, // set fields (incl. nested via dot) $unset: { tempField: "" }, // remove a field $rename: { oldName: "newName" }, // rename a field $inc: { loginCount: 1, score: -5 }, // increment/decrement numbers $mul: { price: 1.1 }, // multiply $min: { lowest: 10 }, $max: { highest: 100 }, // set if lower/higher $currentDate: { lastModified: true } // set to current date }); ``` `$set` (change fields, including nested with dot notation), `$inc` (increment — great for counters), `$unset`, `$rename`, and others modify fields without affecting the rest of the document. ## Array update operators (very important in MongoDB) ```js db.posts.updateOne({ _id: id }, { $push: { tags: "new" }, // add to an array $push: { scores: { $each: [90, 85], $sort: -1, $slice: 5 } }, // add multiple, sort, cap $addToSet: { tags: "unique" }, // add only if NOT already present $pull: { tags: "old" }, // remove matching elements $pop: { queue: -1 }, // remove first (-1) or last (1) $pullAll: { nums: [1, 2] } // remove all listed values }); ``` Array operators are crucial given MongoDB's array-rich documents: `$push` (add), `$addToSet` (add if unique), `$pull` (remove matching), `$pop` (remove ends), and modifiers like `$each`/`$slice` for bulk/bounded arrays. ## Updating array elements (positional operators) ```js // $ — update the FIRST matching array element db.posts.updateOne( { _id: id, "comments.user": "Bob" }, { $set: { "comments.$.text": "Edited" } } // update Bob's comment ); // $[] — update ALL elements; $[identifier] — update elements matching a condition db.users.updateOne({ _id: id }, { $inc: { "scores.$[]": 5 } }); // +5 to all scores ``` Positional operators (`$`, `$[]`, `$[]`) update specific or all array elements — powerful for modifying data inside arrays. ## Why it matters Update operators are essential for **correctly modifying documents** in MongoDB, so understanding them is fundamental everyday knowledge for any data-updating code. The foundational point (covered in basic updates) is that operators modify *specific fields* rather than replacing the whole document — but understanding the **range of operators** is important for expressing the modifications applications need. **Field operators** (`$set`, `$inc` for counters, `$unset`, `$min`/`$max`, etc.) handle field-level changes. Critically, **array update operators** are especially important in MongoDB because documents frequently contain arrays (a core part of the document model), and `$push` (add), `$addToSet` (add-if-unique), `$pull` (remove matching), `$pop`, and modifiers (`$each`, `$slice` for bounded arrays) are how you correctly manipulate array data — a frequent need that's central to working with MongoDB's document structure. **Positional operators** (`$`, `$[]`, `$[]`) for updating specific or all array elements are powerful for modifying data nested within arrays — handling cases like editing a specific comment in an array. Understanding these operators enables the **atomic, targeted modifications** MongoDB excels at (modifying parts of a document in one atomic operation). Since updating documents — especially their arrays — is a constant need, and since MongoDB's rich update operators (field, array, and positional) are how you express these modifications correctly and atomically, understanding update operators is essential, frequently-applied MongoDB knowledge, particularly the array operators which are central to working with MongoDB's array-rich documents and a common area where knowing the right operator makes complex modifications clean and atomic.