Can Rails Migrations be used to convert data?

What you’re trying to do is possible, and I would say the correct thing to do.

You need, though, to reload the column info for the model classes you’re updating in the migration, so that Rails knows about the new columns. Try this:

def.self up
    add_column :users, :age_text, :string

    User.reset_column_information 

    users = User.find(:all)

    users.each do |u|
       u.age_text = convert_to_text(u.age)
       u.save
    end
end

On a separate note, please note that if your table is large, doing updates one by one will take a looong time.. Be careful with that.