MongoDB Field Update Operators

These operators do the actual work inside updateOne() / updateMany() — they describe how a field's value should change.

Operator	Description
$set	Sets a field's value (creates the field if it doesn't exist)
$unset	Removes a field entirely
$inc	Increments a numeric field by a given amount
$min	Only updates if the new value is less than the current one
$max	Only updates if the new value is greater than the current one
$mul	Multiplies a numeric field by a given amount
$rename	Renames a field
$setOnInsert	Sets a value only when an upsert results in an insert
$currentDate	Sets a field to the current date or timestamp

$set

db.employees.updateOne({ empid: 202 }, { $set: { email: 'rajangupta@gmail.com' } })

$unset

The value you give the field doesn't matter — 1 is the convention. It's a no-op (no error) if the field doesn't already exist.

db.employees.updateMany({}, { $unset: { reporting: 1 } })
db.employees.updateMany({}, { $unset: { name: 1, allmarks: 1 } }) // multiple fields at once

$inc

db.employees.updateOne({ empid: 201 }, { $inc: { salary: 5000 } })   // +5000
db.employees.updateOne({ empid: 202 }, { $inc: { salary: -1000 } })  // -1000

If the field doesn't exist, $inc creates it and sets it to the given amount.

$min / $max

db.employees.updateOne({ empid: 201 }, { $min: { salary: 45000 } })
// only applies if the current salary is greater than 45000

db.employees.updateOne({ empid: 201 }, { $max: { salary: 50000 } })
// only applies if the current salary is less than 50000

$mul

db.employees.updateOne({ empid: 202 }, { $mul: { salary: 2 } })
// doubles the salary

If the field doesn't exist yet, $mul creates it and sets it to 0.

$rename

db.employees.updateOne({ empid: 202 }, { $rename: { name: 'fname' } })
db.employees.updateMany({}, { $rename: { fname: 'name' } }) // rename it back everywhere

Silently does nothing if the field doesn't exist — no error is thrown.

$setOnInsert

Only takes effect when an upsert results in a brand-new document — it has zero effect on documents that already existed.

db.employees.updateOne(
  { empid: 210 },
  { $setOnInsert: { fathername: "Father", mothername: "Mother" } },
  { upsert: true }
)

$currentDate

Sets a field to the current date, either as a Date object or a BSON Timestamp.

db.employees.updateOne({ empid: 201 }, { $currentDate: { doj: true } })
// sets doj to a Date

db.employees.updateOne({ empid: 202 }, { $currentDate: { doj: { $type: 'timestamp' } } })
// sets doj to a Timestamp instead

Common Errors & Troubleshooting

• $inc on a non-numeric field — $inc errors if the field currently holds a non-numeric value (like a string); it can only increment numbers.
• Combining $set and $currentDate on the same field in one update — you can't target the same field with two different update operators in a single update document; pick one.
• $rename silently doing nothing — if the source field name doesn't exist in the document, $rename just no-ops without any error or warning — double-check for typos in the field name if a rename doesn't seem to take effect.

Key Takeaways

• $set/$unset handle adding and removing fields; $inc/$mul handle in-place math.
• $min/$max are "conditional sets" — they only write when the new value beats the old one.
• $setOnInsert is the only operator that behaves differently for inserts vs. updates during an upsert.