How to update documents in MongoDB with updateOne and updateMany

Always use update operators

MongoDB updates are surgical by default. The second argument to updateOne or updateMany must be an object containing update operators like $set. Passing a plain document without operators replaces the entire matched document — use replaceOne intentionally when that is what you want, not accidentally inside an update call.

// $set — add or overwrite specific fields, leave others untouched
await db.collection('products').updateOne(
  { name: 'Mouse' },
  { $set: { price: 34.99, updatedAt: new Date() } }
)

// $unset — remove a field from the document
await db.collection('products').updateOne(
  { name: 'Mouse' }, { $unset: { legacyCode: '' } }
)

// $inc — atomic server-side increment, safe under concurrency
await db.collection('products').updateOne(
  { name: 'Keyboard' }, { $inc: { stock: -1 } }
)

// $push — append an element to an array field
await db.collection('posts').updateOne(
  { _id: postId }, { $push: { tags: 'mongodb' } }
)

// $pull — remove elements matching a condition from an array
await db.collection('posts').updateOne(
  { _id: postId }, { $pull: { tags: 'draft' } }
)

updateMany and upsert

// Update all documents matching the filter
await db.collection('products').updateMany(
  { stock: 0 }, { $set: { discontinued: true } }
)

// upsert: insert if no match, update if found
await db.collection('settings').updateOne(
  { userId: 'u1' }, { $set: { theme: 'dark' } }, { upsert: true }
)