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

Rails
1

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] = [Rails::Generator::GeneratedAttribute.new
(“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 Rails::Generator::GeneratedAttribute.new allows just two
parameters.

Any ideas?
Thanks,
Bernd