module Rails::Generators::Migration

def create_migration(destination, data, config = {}, &block) 

def create_migration(destination, data, config = {}, &block)
  action Rails::Generators::Actions::CreateMigration.new(self, destination, block || data.to_s, config)
end

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 version, migration file name, migration class name are

to the destination file name.
to the default template method is that the migration version is appended
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)
  context = instance_eval("binding")
  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(context)
  end
  Rails::Generators.add_generated_file(file)
end

def set_migration_assigns!(destination) 

def set_migration_assigns!(destination)
  destination = File.expand_path(destination, destination_root)
  migration_dir = File.dirname(destination)
  @migration_number     = self.class.next_migration_number(migration_dir)
  @migration_file_name  = File.basename(destination, ".rb")
  @migration_class_name = @migration_file_name.camelize
end