A migration I created in a Rails 5 application had
class CreateVariableKeys < ActiveRecord::Migration[5.0]
It is a class method of
ActiveRecord::Migration and is defined here.
It allows us to select the version of migrations we wish to use between
5.0. The method throws a:
"Unknown migration version ... "
error if an incompatible version is passed as an argument.
Production ready versions of
ActiveRecord don’t have that method so it should go away as soon as Rails 5 goes out of beta.