Update Operations

Updates modify existing documents without replacing the entire document (unless you use replaceOne). updateOne changes the first match; updateMany changes every match. Both require update operators like $set — passing a plain object without operators replaces fields incorrectly in some contexts.

updateOne() and updateMany()

Real-life example: updateOne is like editing one student's grade in the register. updateMany is like raising every product price by 10% in one go.

updateOne — single document

use rishtaara

// Increase stock and stamp updatedAt
db.products.updateOne(
  { name: "Wireless Mouse" },
  {
    $set: { updatedAt: new Date() },
    $inc: { stock: 50 }
  }
)

// Return value:
// {
//   acknowledged: true,
//   matchedCount: 1,    // documents matching the filter
//   modifiedCount: 1,   // documents actually changed
//   upsertedCount: 0,
//   upsertedId: null
// }

// No match — matchedCount: 0, modifiedCount: 0
db.products.updateOne(
  { name: "Nonexistent Product" },
  { $set: { price: 999 } }
)

updateMany — all matches

// Apply 15% discount to all stationery
db.products.updateMany(
  { category: "stationery" },
  { $mul: { price: 0.85 }, $set: { onSale: true, updatedAt: new Date() } }
)

// Deactivate all students who graduated
db.students.updateMany(
  { grade: { $gt: 12 } },
  { $set: { active: false, graduatedAt: new Date() } }
)

// Add a tag to every electronics product
db.products.updateMany(
  { category: "electronics" },
  { $addToSet: { tags: "tech" } }
)

replaceOne() — full document swap

replaceOne() removes the existing document and inserts the replacement — except _id stays the same. Use only when you truly want to overwrite every field.

Real-life example: replaceOne is like tearing out a page and writing a completely new entry — not just correcting one line.

In 99% of production updates, use updateOne/updateMany with $set, $inc, and other operators. replaceOne is for full document replacement only.

replaceOne example

// Current document has many fields
db.settings.findOne({ key: "app_config" })
// { _id: ..., key: "app_config", theme: "dark", lang: "en", version: 3 }

// replaceOne overwrites ALL fields (except _id)
db.settings.replaceOne(
  { key: "app_config" },
  { key: "app_config", theme: "light", version: 4 }
)
// theme, lang changed; lang field is GONE unless you include it

// Prefer $set for partial updates in production
db.settings.updateOne(
  { key: "app_config" },
  { $set: { theme: "light", version: 4, updatedAt: new Date() } }
)

Upsert — update or insert

Setting { upsert: true } creates a new document when no match is found. Perfect for settings, counters, and idempotent sync operations.

Real-life example: Upsert is like 'update my seat assignment if I have one, otherwise create a new seat' — one operation handles both cases.

Upsert patterns

// Settings key-value store
db.settings.updateOne(
  { key: "maintenance_mode" },
  { $set: { value: false, updatedAt: new Date() } },
  { upsert: true }
)
// Creates { key: "maintenance_mode", value: false, ... } if missing

// User preference sync
db.user_prefs.updateOne(
  { userId: ObjectId("665a1b2c3d4e5f6789012345"), pref: "theme" },
  { $set: { value: "dark", updatedAt: new Date() } },
  { upsert: true }
)

// Return value when upsert creates:
// { matchedCount: 0, modifiedCount: 0, upsertedCount: 1, upsertedId: ObjectId("...") }

Legacy update() and intro to update operators

The legacy update() method is deprecated. Use updateOne, updateMany, or replaceOne instead. Update operators modify specific fields — detailed coverage of $set, $inc, $push, and more comes in the Update Operators module.

Real-life example: $set is like correcting one cell in a spreadsheet. $inc is like adding 10 to a stock count without reading the current value first.

Never pass a replacement document to updateOne without operators — use replaceOne if that is your intent.
upsert: true is powerful for config and counter collections.
$inc is atomic — safe for concurrent stock decrements.
Full operator reference ($push, $pull, $addToSet, etc.) is covered in the next module.

Legacy vs modern update

// ❌ Legacy (deprecated)
db.products.update({ name: "Mouse" }, { $set: { price: 799 } })
db.products.update({ category: "sale" }, { $set: { discounted: true } }, { multi: true })

// ✅ Modern
db.products.updateOne({ name: "Mouse" }, { $set: { price: 799 } })
db.products.updateMany({ category: "sale" }, { $set: { discounted: true } })

Quick intro — $set and $inc

// $set — assign or overwrite field values
db.students.updateOne(
  { name: "Asha Khan" },
  { $set: { grade: 11, "address.city": "Delhi", updatedAt: new Date() } }
)

// $inc — increment numeric fields (atomic)
db.products.updateOne(
  { name: "Wireless Mouse" },
  { $inc: { stock: -1, soldCount: 1 } }  // sell one unit
)

// Combine operators in one update
db.students.updateOne(
  { name: "Rahul Sharma" },
  {
    $set: { lastLogin: new Date() },
    $inc: { loginCount: 1 }
  }
)