More ActiveRecord Pilfering

After pilfering Aaron Patterson’s to_dot idea last week, I decided to pilfer another good idea he recently added to ActiveRecord, reversible migrations.

The idea behind reversible migrations is that in many cases, the library can know exactly how to reverse the migration you are using. For example, if you are using create_table, it can be reversed by using drop_table. And if you are using create_index followed add_column, it can reverse it by calling drop_column and then drop_index. Basically, all reversible migrations can be reversed by applying the reverse of each action in reverse order.

Sequel implements reversible migrations using a change block inside of a Sequel.migration block:

Sequel.migration do
  change do
    create_table(:artists) do
      primary_key :id
      String :name, :null=>false
    end
  end
end

This will automatically create the equivalent of:

Sequel.migration do
  up do
    create_table(:artists) do
      primary_key :id
      String :name, :null=>false
    end
  end

  down do
    drop_table(:artists)
  end
end

There is no support for reversible migrations using the historical (but still supported) usage of creating a subclass of Sequel::Migration.

The following Database methods are handled and can be reversed successfully:

• +create_table+
• +add_column+
• +add_index+
• +rename_column+
• +rename_table+
• +alter_table+ (supporting the following methods in the +alter_table+ block):
  • +add_column+
  • +add_constraint+
  • +add_foreign_key+ (with a symbol, not an array)
  • +add_primary_key+ (with a symbol, not an array)
  • +add_index+
  • +add_full_text_index+
  • +add_spatial_index+
  • +rename_column+

Usage of any other method in the change block will result in a down block created that raises a Sequel::Error (basically making the migration irreversible).