Update MongoDB collection using $toLower

Question

I have an existing MongoDB collection containing user names. The user names contain both lower case and upper case letters.

I want to update all the user names so th

Answer 1

Starting Mongo 4.2, db.collection.update() can accept an aggregation pipeline, finally allowing the update of a field based on its own value:

// { username: "Hello World" }
db.collection.update(
  {},
  [{ $set: { username: { $toLower: "$username" } } }],
  { multi: true }
)
// { username: "hello world" }

• The first part {} is the match query, filtering which documents to update (in this case all documents).

• The second part [{ $set: { username: { $toLower: "$username" } } }], is the update aggregation pipeline (note the squared brackets signifying the use of an aggregation pipeline):

  • $set is a new aggregation operator which in this case modifies the value for "username".
  • Using $toLower, we modify the value of "username" by its lowercase version.

• Don't forget { multi: true }, otherwise only the first matching document will be updated.