Can I rollback using change_column method in Rails 4 and above?

Question

Is change_column method able to rollback when it is used in migrations in Rails 4 and above?

Here is an example:

def change
  change_column etc
end

Should I use up and down methods instead?

Answer 1

change_column cannot be reverted by default. Rails cannot know the column definition before the migration, so it cannot roll back to this original definition.

Therefor starting with Rails 4, you can provide Rails with the necessary information. That's what the reversible method is for:

def change
  reversible do |dir|
    dir.up do
      change_column :users, :score, :decimal, precision: 6, scale: 2
    end
    dir.down do
      change_column :users, :score, :decimal, precision: 4, scale: 2
    end
  end
end

At first glance, this seems more complicated than using up and down. The advantage is, that you can mix it with reversible migrations.
If i.e. you add a bunch of fields to a table and have to change few fields along, you can use change_table and add_column together with reversible to have a clean and compact migration:

def change
  change_table :users do |t|
    t.string :address
    t.date :birthday
    # ...
  end

  reversible do |dir|
    dir.up do
      change_column :users, :score, :decimal, precision: 6, scale: 2
    end
    dir.down do
      change_column :users, :score, :decimal, precision: 4, scale: 2
    end
  end
end