fail to update documents with cursor.forEach

This Meteor client code does not update the documents found as expected. The console.log(res) prints '0' when there are documents to be updated.
Why and how to fix it? Thanks

MyCollection.find({
  class: 'check-filter'
}).forEach((obj) => {
  MyCollecction.update({
    obj
  }, {
    $set: {
      class: ''
    }
  }, (err, res) => {
    if (!err) {
      console.log(res);
    }
  });
});