How to limit number of updating documents in mongodb

Unfortunately the workaround you have is the only way to do it AFAIK. There is a boolean flag multi which will either update all the matches (when true) or update the 1st match (when false).

You can use:

db.collection.find().limit(NUMBER_OF_ITEMS_YOU_WANT_TO_UPDATE).forEach(
    function (e) {
        e.fieldToChange = "blah";
        ....
        db.collection.save(e);
    }
);

(Credits for forEach code: MongoDB: Updating documents using data from the same document)

What this will do is only change the number of entries you specify. So if you want to add a field called "newField" with value 1 to only half of your entries inside "collection", for example, you can put in

db.collection.find().limit(db.collection.count() / 2).forEach(
    function (e) {
        e.newField = 1;
        db.collection.save(e);
    }
);

If you then want to make the other half also have "newField" but with value 2, you can do an update with the condition that newField doesn't exist:

db.collection.update( { newField : { $exists : false } }, { $set : { newField : 2 } }, {multi : true} );

The solutions that iterate over all objects then update them individually are very slow.

Retrieving them all then updating simultaneously using $in is more efficient.

ids = People.where(firstname: 'Pablo').limit(10000).only(:_id).to_a.map(&:id)
People.in(_id: ids).update_all(lastname: 'Cantero')

The query is written using Mongoid, but can be easily rewritten in Mongo Shell as well.

Using forEach to individually update each document is slow. You can update the documents in bulk using

ids = db.collection.find(<condition>).limit(<limit>).map(
    function(doc) {
        return doc._id;
    }
);
db.collection.updateMany({_id: {$in: ids}}, <update>})