Add model virtual attribute

Bad Smell

<% form_for @user do |f| %>
  <%= text_field_tag :full_name %>
<% end %>

class UsersController < ApplicationController
  def create
    @user = User.new(params[:user])
    @user.first_name = params([:full_name]).split(' ', 2).first
    @user.last_name = params([:full_name]).split(' ', 2).last
    @user.save
  end
end

In this example, the user form contains a full_name attribute, but we want to save the first_name and last_name in table, so we assign the first_name an last_name for @user object in controller by splitting the full_name.

But assigning attributes for model is not the job of controller, you should let model to handle this by adding virtual attribute.

Refactor

class User < ActiveRecord::Base
  def full_name
    [first_name, last_name].join(' ')
  end

  def full_name=(name)
    split = name.split(' ', 2)
    self.first_name = split.first
    self.last_name = split.last
  end
end

<% form_for @user do |f| %>
  <%= f.text_field :full_name %>
<% end %>

class UsersController < ApplicationController
  def create
    @user = User.create(params[:user])
  end
end

Now User model adds the virtual attribute full_name=, whose function is to assign first_name and last_name. It's cool because we can use only one line to create a user, and User model will split the full_name and assign first_name and last_name by itself.