Rails Migration - Non-Static Default Value

Question

I had a quick question about setting the default value of a column in a Rails migration to a non-static value.

I have a "Users" table that already has the columns id, first_name, and last_name, and I want to add a new column called "username" that defaults to the user's last name. This is what I currently have:

class AddLoginToUsers < ActiveRecord::Migration
  def change
    add_column :users, :username, :string, :default => :last_name
  end
end

Clearly, this does not produce the intended result (it defaults to the string "last_name"). How would I go about setting the default to the row's last_name value?

Thanks in advance.

Answer 1

Not sure you can do that, you could instead add a before_validation or before_save callback in your User model:

class User < ActiveRecord::Base
  before_validation :create_username_if_missing

  private

  def create_username_if_missing
    self.username = last_name unless username
  end
end

And update all exisiting users after adding that callback:

User.where("username is null").each do |user|
  user.update_attributes(username: user.last_name)
end