Using Virtual Attributes in Rails 3

In Rails, virtual attributes allow you to create form fields that do not map directly to the database.

This can be useful in a variety of situations and can help you customize your interface to make it more intuitive and user-friendly.

In this short tutorial I will show you how to create three text fields to allow a user to enter a date (day, month, year), then combine the values entered into these ‘virtual’ fields and save the new value to the database.

Let’s start off by creating a new project called birthdays:

rails new birthdays
cd birthdays
bundle install

Now let’s make use of Rails’ scaffolding generator:

rails generate scaffold Birthdays name:string date:string
rake db:migrate
rails s

If you navigate to http://localhost/birthdays/ you should now see your newly created Rails app, with the option to add a “New Birthday”.

But, before we click that, let’s make a couple of changes to the code that Rails generated for us.

Open the _form partial (located in /app/views/birthdays/) in your favourite editor and change this:

<div class="field">
  <%= f.label :date %><br />
  <%= f.text_field :date %>

into this:

<div class="field">
  <%= f.label :day %><br />
  <%= f.text_field :day %>-
  <%= f.text_field :month %>-
  <%= f.text_field :year %>

Now open up your model birthday.rb. You should see something like this:

class Birthday < ActiveRecord::Base
  attr_accessible :date, :name

For the virtual attributes to work, we need to create getter and setter methods for day, month and year. We then need to create a before_validation callback, to ensure that the correct value is assigned to the model’s date attribute when the form is submitted.

Initially I used :attr_accessor to create the getter and setter methods, but this has a caveat: the getter methods work just fine when we’re submitting the form to create a record, however when we are reading a record from the database and using the same form to edit it, the date is displayed as blank.

To get around this, I check if date is set. If it is, I split it and return the appropriate element. If it isn’t, I proceed as normal.

Here’s the complete code:

class Birthday < ActiveRecord::Base
  attr_accessible :date, :name, :day, :month, :year
  before_validation :make_date

  def day
    (date.nil?)? @day : date.split("-")[0]

  def month
   (date.nil?)? @month : date.split("-")[1]

  def year
    (date.nil?)? @year : date.split("-")[2]

  def day=(val)
    @day = val

  def month=(val)
    @month = val

  def year=(val)
    @year = val

  def make_date
    birthday = [@day, @month, @year].join("-") = (birthday == "--")? "" : birthday

And that’s all there is to it. Now when you create a new record, you’ll be able to enter the day, month and year of the birthday individually, yet when you look at the database, the date will be stored in one column.