The migration examples in the Rails Edge Guide are all like this
class MyMigration < ActiveRecord::Migration[5.0] ... end
What’s that [5.0]? It can’t possibly be Ruby. Or is it?
With a little experimenting in rails c it’s easy to understand that [ is a class method of ActiveRecord::Migration. It’s defined in activerecord-5.0.0.beta1/lib/active_record/migration.rb and it accepts the 4.2 and 5.0 arguments. It allows us to select which version of the migrations we want to use. Production ready versions of ActiveRecord don’t have that method so it should go away as soon as Rails 5 goes out of beta.