Can't run a migration adding userstamp to an existing table Rails - ruby-on-rails

Question

I am using the gem blamer to add a userstamp into some tables that are already created. I tried to add userstamps while creating a new table and it worked.But I need to add to existing tables and I don't know what to write in the migration. This gem is like others that add userstamp but only this one seems working in rails 6.
class User < ActiveRecord::Base
cattr_accessor :current_user
end
class ApplicationController < ActionController::Base
before_action :set_userstamp
def set_userstamp
User.current_user = User.find(session[:user_id])
end
end
class AddUserstampsToIllustrators < ActiveRecord::Migration[6.0]
def change
add_column :illustrators, :userstamps
end
end
I got the following error:
== 20201106193035 AddUserstampsToIllustrators: migrating ======================
-- add_column(:illustrators, :userstamps)
rails aborted!
StandardError: An error has occurred, this and all later migrations canceled:
wrong number of arguments (given 2, expected 3)
/home/morganaborges/code/MorganaTBorges/bamboleio_project/db/migrate/20201106193035_add_userstamps_to_illustrators.rb:3:in `change'
/home/morganaborges/code/MorganaTBorges/bamboleio_project/bin/rails:9:in `<top (required)>'
/home/morganaborges/code/MorganaTBorges/bamboleio_project/bin/spring:15:in `<top (required)>'
./bin/rails:3:in `load'
./bin/rails:3:in `<main>'
Caused by:
ArgumentError: wrong number of arguments (given 2, expected 3)
/home/morganaborges/code/MorganaTBorges/bamboleio_project/db/migrate/20201106193035_add_userstamps_to_illustrators.rb:3:in `change'
/home/morganaborges/code/MorganaTBorges/bamboleio_project/bin/rails:9:in `<top (required)>'
/home/morganaborges/code/MorganaTBorges/bamboleio_project/bin/spring:15:in `<top (required)>'
./bin/rails:3:in `load'
./bin/rails:3:in `<main>'
Tasks: TOP => db:migrate
(See full trace by running task with --trace)

Answer 1

Looking at https://github.com/infused/blamer , by default the two columns are created_by and updated_by with type integer. So...
add_column :illustrators, :created_by, :integer
add_column :illustrators, :updated_by, :integer
(integer being used in https://github.com/infused/blamer/blob/master/lib/blamer/userstamp.rb#L47 )

Answer 2

ArgumentError: wrong number of arguments (given 2, expected 3) you pass two arguments but you need to pass column type as the third argument. Here is the documentation with a list of types.
add_column :illustrators, :userstamps, :string
# or
add_column :illustrators, :userstamps, :datetime
# etc.
Take a look at the examples section.