module Rails::Generators::Migration

def migration_template(source, destination, config = {})

migration_template "migration.rb", "db/migrate/add_foo_to_bar.rb"

available as instance variables in the template to be rendered.
The migration number, migration file name, migration class name are

to the destination file name.
to the default template method is that the migration number is prepended
Creates a migration template at the given destination. The difference 
def migration_template(source, destination, config = {})
  source = File.expand_path(find_in_source_paths(source.to_s))
  set_migration_assigns!(destination)
  dir, base = File.split(destination)
  numbered_destination = File.join(dir, ["%migration_number%", base].join("_"))
  file = create_migration numbered_destination, nil, config do
    ERB.new(::File.binread(source), trim_mode: "-", eoutvar: "@output_buffer").result(binding)
  end
  Rails::Generators.add_generated_file(file)
end