Defines a versioning change.

A versioning change is used to make small changes to data of a certain type. They are used within a Versioning.Schema. Changes should attempt to be as focused as possible to ensure complexity is kept to a minimum.

Example

defmodule MyApp.Cheanges.PostStatusChange do
  use Versioning.Change

  @desc "The 'active' attribute has been changed in favour of the 'status' attribute"

  @impl Versioning.Change
  def down(versioning, _opts) do
    case Versioning.pop_data(versioning, "status") do
      {:active, versioning} -> Versioning.put_data(versioning, "active", true)
      {_, versioning} -> Versioning.put_data(versioning, "active", false)
    end
  end

  @impl Versioning.Change
  def up(versioning, _opts) do
    case Versioning.pop_data(versioning, "active") do
      {true, versioning} -> Versioning.put_data(versioning, "status", "active")
      {false, versioning} -> Versioning.put_data(versioning, "status", "hidden")
      {_, versioning} -> versioning
    end
  end
end

The above change module represents us modifying our Post data to support a new attribute - status - which replaces the previous active attribute.

When changing data "down", we must remove the status attribte, and replace it with a value that represents the previous active attribute. When changing data "up", we must remove the active attribute and replace it with a value that represents the new status attribute.

Descriptions

Change modules can optionally include a @desc module attribute. This will be used to describe the changes made in the change module when constructing changelogs. Please see the Versioning.Changelog documentation for more information on changelogs.