validates attr específico

index.htm.erb

  <%= form_for @user, :html => { :class => 'ui form attached fluid segment cadastro', :onsubmit => 'cadastro()' } do |f| %>

  <% if @user.errors[:email].any? %>
  <div class="ui error message visible">
    <div class="header"><i class="icon attention"></i>Atenção</div>
    <p><%= @user.errors[:email] %></p>
  </div>
  <% end %>

  <div class="two fields">
    <div class="field <%= error_tag @user, :name %>">
      <label>Seu Nome</label>
      <%= f.text_field :name, placeholder: "Seu nome" %>
    </div>
    <div class="field <%= error_tag @user, :second_name %>">
      <label>Sobrenome</label>
      <%= f.text_field :second_name, placeholder: "Sobrenome" %>
    </div>
  </div>

  <div class="field <%= error_tag @user, :gender %>">
    <label>Sexo</label>
    <div class="ui fluid selection dropdown">
      <div class="text"><p style="color:#ccc">Selecione</p></div>
      <i class="dropdown icon"></i>
      <%= f.text_field :gender, type: 'hidden' %>
      <div class="menu">
        <div class="item" data-value="masculino">Masculino</div>
        <div class="item" data-value="feminino">Feminino</div>
      </div>
    </div>
  </div>

  <div class="field <%= error_tag @user, :email %>">
    <label>E-mail</label>
    <%= f.text_field :email, placeholder: "E-mail" %>
  </div>

  <div class="field <%= error_tag @user, :password %>">
    <label>Senha</label>
    <%= f.password_field :password, placeholder: "Senha" %>
  </div>

  <div class="inline field <%= error_tag @user, :terms %>">
    <div class="ui checkbox">
      <%= f.check_box :terms %>
      <label>Eu aceito os termos e condições <a href="#" class="item floated left terms">Termos</a></label>
    </div>
  </div>
  <%= f.submit 'Cadastrar', class: 'ui teal submit button' %>

  <% end %>

  <!-- Avisos -->

  <% if @user.errors.any? %>
  <div class="ui bottom attached error message">
    <i class="icon attention"></i>
    <%= @user.errors.full_messages.first do |message| message end %>
  </div>
  <% end %>

user.rb

validates :email, presence: true, if: :email_presence
attr_accessor :email_presence?
def email_presence
   self.email_presence = true if self.email.blank?
end

view

<% if @user.email_presence? %>
  <div class="ui error message visible">
  <div class="header"><i class="icon attention"></i>Atenção</div>
    <p><%= I18n.t(:taken, scope: 'errors.messages') %></p>
  </div>
<% endif %>