Rails radio buttons and required boolean attributes

How to deal with required boolean attributes and radio buttons in Ruby on Rails.

Lately, I was trying to create a form for one of my models which had a required boolean attribute.

The model

class Foo < ActiveRecord::Base

  validates_presence_of :a_flag

end

The form

<h1>Foos#new</h1>

<%= form_for @foo do |form| %>
    <% if @foo.errors %>
        <% @foo.errors.full_messages.each do |message| %>
            <%= message %>
            <br />
        <% end %>
    <% end %>
    <table>
        <tr>
            <td><%= form.label 'a_flag_true', 'Yes' %></td>
            <td><%= form.radio_button :a_flag, true %></td>
        </tr>

        <tr>
            <td><%= form.label 'a_flag_false', 'No' %></td>
            <td><%= form.radio_button :a_flag, false %></td>
        </tr>
    </table>

    <%= form.submit 'Answer' %>
<% end %>

So far so good. Back to the browser now, everything seemed fine till the moment I tried to submit the No value.

I googled around to find some answers on this and some suggested that I should use the inclusion validator. And so did I.

The updated model

class Foo < ActiveRecord::Base

  validates :a_flag,
            :inclusion => { :in => [true, false] }

end

Back to the browser everything seemed fine. I could submit both Yes and No values but there was a problem when I was trying to submit the form without selecting a value.

The error message was not quite what I wanted… And for sure, I wanted to find a solution that won’t make me override default error messages.

And then I thought, what if I forced the presence validation only when the value was nil?

The model

class Foo < ActiveRecord::Base

  validates :a_flag,
            :presence => { :if => 'a_flag.nil?' }

end

I could now submit both values and the desired message was showing up if no selection was made.

Done. Problem solved.