'find or create by' Rails

_form.html.erb

<%= form_for(@quote) do |f| %>
  <% if @quote.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(@quote.errors.count, "error") %> prohibited this quote from being saved:</h2>

      <ul>
      <% @quote.errors.full_messages.each do |msg| %>
        <li><%= msg %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <div class="field">
    <%= f.label :text, "Quote:" %><br />
    <%= f.text_area :text %>
  </div>

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

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

  <div class="field">
    <%= f.label :tag_names, "Tags" %><br />
    <%= f.text_field :tag_names %>
  </div>
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

quote.rb

class Quote < ActiveRecord::Base
  has_many :taggings, :dependent => :destroy
  has_many :tags, :through => :taggings
  
  after_save :assign_tags
    
  attr_accessor :tag_names
  def tag_names
    @tag_names || tags.map(&:name).join(', ')
  end
  
  private 
  
 
  def assign_tags
    if @tag_names
      self.tags = @tag_names.split(/,+/).map do |name|
        Tag.find_or_create_by_name(name)
      end
    end
  end
end