MongoDb - MongoDB update operations

1. Overview

MongoDB provides three main update methods:

• updateOne(filter, update, options) → updates the first matching document.

• updateMany(filter, update, options) → updates all matching documents.

• replaceOne(filter, replacement, options) → replaces an entire document.

All updates are performed in-place (only the specified fields are modified) unless you use replaceOne.

2. Update Operators

Updates use operators to specify how documents should change.

Common Update Operators:

• $set → set a field value

• $unset → remove a field

• $inc → increment/decrement a number

• $mul → multiply a field

• $rename → rename a field

• $push → append to an array

• $addToSet → append only if not already present

• $pull → remove from an array

• $pop → remove first/last element of an array

3. Examples

a) Update One Document

db.users.updateOne(
  { name: "Alice" },           // filter
  { $set: { age: 26 } }        // update
)

• Finds the first user named Alice and sets age = 26.

b) Update Multiple Documents

db.users.updateMany(
  { age: { $lt: 25 } },
  { $set: { status: "young" } }
)

• Adds status: "young" to all users younger than 25.

c) Increment a Value

db.users.updateOne(
  { name: "Bob" },
  { $inc: { loginCount: 1 } }
)

• Increases Bob’s loginCount by 1.

d) Remove a Field

db.users.updateOne(
  { name: "Charlie" },
  { $unset: { email: "" } }
)

• Removes the email field from Charlie’s document.

e) Array Updates

db.users.updateOne(
  { name: "Alice" },
  { $push: { hobbies: "cycling" } }
)

db.users.updateOne(
  { name: "Alice" },
  { $pull: { hobbies: "reading" } }
)

• $push adds "cycling" to Alice’s hobbies array.

• $pull removes "reading" from the array.

f) Replace Entire Document

db.users.replaceOne(
  { name: "David" },
  { name: "David", age: 30, country: "USA" }
)

• Replaces David’s whole document with the new one.

• ⚠️ Any fields not included are lost.

4. Options

• upsert: true → Insert a new document if none matches.

Example:

db.users.updateOne(
  { name: "Eve" },
  { $set: { age: 29 } },
  { upsert: true }
)

• If Eve exists → update her age.

• If not → insert a new document { name: "Eve", age: 29 }.

5. Verify Updates

Check updated documents:

db.users.find({ name: "Alice" })