Holds common methods for migrations. It assumes that migrations have the [0-9]*_name format and can be used by other frameworks (like Sequel) just by implementing the next migration version method.

Methods
C
M
S
Attributes
[R] migration_class_name
[R] migration_file_name
[R] migration_number
Instance Public methods
create_migration(destination, data, config = {}, &block)

Source: show | on GitHub 

# File railties/lib/rails/generators/migration.rb, line 35
def create_migration(destination, data, config = {}, &block)
  action Rails::Generators::Actions::CreateMigration.new(self, destination, block || data.to_s, config)
end
migration_template(source, destination, config = {})

Creates a migration template at the given destination. The difference to the default template method is that the migration version is appended to the destination file name.

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

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

Source: show | on GitHub 

# File railties/lib/rails/generators/migration.rb, line 56
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("_"))

  create_migration numbered_destination, nil, config do
    if ERB.instance_method(:initialize).parameters.assoc(:key) # Ruby 2.6+
      ERB.new(::File.binread(source), trim_mode: "-", eoutvar: "@output_buffer").result(context)
    else
      ERB.new(::File.binread(source), nil, "-", "@output_buffer").result(context)
    end
  end
end
set_migration_assigns!(destination)

Source: show | on GitHub 

# File railties/lib/rails/generators/migration.rb, line 39
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