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 they only contain lower case letters.

I have tried this script, but it didn't work

db.myCollection.find().forEach(  function(e) {  e.UserName = $toLower(e.UserName);  db.myCollection.save(e);  } ) 

Answer 1

MongoDB does not have a concept of $toLower as a command. The solution is to run a big for loop over the data and issue the updates individually.

You can do this in any driver or from the shell:

db.myCollection.find().forEach(   function(e) {     e.UserName = e.UserName.toLowerCase();     db.myCollection.save(e);   } ) 

You can also replace the save with an atomic update:

db.myCollection.update({_id: e._id}, {$set: {UserName: e.UserName.toLowerCase() } }) 

Again, you could also do this from any of the drivers, the code will be very similar.

---

EDIT: Remon brings up a good point. The $toLower command does exist as part of the aggregation framework, but this has nothing to do with updating. The documentation for updating is here.

Answer 2

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.