generate a migration (from a plugin) to add a column with a default value to an existing table

I add a column to an existing table via the generator with the following action:

  def cmi_local_assigns     returning(assigns = {}) do       assigns[:migration_action] = "add"       assigns[:class_name] = "cmi_add_avatar_to_user#{class_name}"       assigns[:table_name] = args[1]       assigns[:attributes] = [ ("user_avatar", "string")]     end   end

This works fine, bug now I want to add a default value for the newly created column. How is this possible? The allows just two parameters.

Any ideas? Thanks, Bernd