Rails: how to join two models through two different relations?

I have two models: Saft (a magazine) and Keyword. Each "Saft" is defined by a series of keywords, but also has a title, which is always one of its keywords. The Saft and Keyword models are connected through a HABTM join table in order to pull all the keywords and now I am trying to pull the title from the keywords table onto the saft/show.html.erb, too. I am trying to use the class_name option in order to pull the title. Therefore I created the Edition model.

class Saft < ActiveRecord::Base
  attr_accessible :colour, :cover_alt, :description, :number, :short
  has_and_belongs_to_many :keywords, :join_table => "safts_keywords"
  has_one :title, :through => :edition, :class_name => "keyword"
  has_one :edition
end

class Keyword < ActiveRecord::Base
  attr_accessible :word, :description
  has_and_belongs_to_many :safts, :join_table => "safts_keywords"
  belongs_to :issue, :through => :edition, :class_name => "saft"
end

class Edition < ActiveRecord::Base
  attr_accessible :saft_id, :keyword_id
  belongs_to :saft
  belongs_to :keyword
end

class SaftsController < ApplicationController 
  def show
    @saft = Saft.find(params[:id])
  end

show.html.erb
    <%= @saft.title.upcase %>

I get the following error:

Started GET "/safts/2" for 127.0.0.1 at Sat Feb 10 17:31:28 +0100 2018
Connecting to database specified by database.yml
Processing by SaftsController#show as HTML
  Parameters: {"id"=>"2"}
  Saft Load (1.8ms)  SELECT `safts`.* FROM `safts` WHERE `safts`.`id` = ? LIMIT 1  [["id", "2"]]
  Image Load (0.3ms)  SELECT `images`.* FROM `images` WHERE `images`.`saft_id` = 2
  Rendered safts/show.html.erb within layouts/public (35.0ms)
Completed 500 Internal Server Error in 103ms

ActionView::Template::Error (uninitialized constant Saft::keyword):
    29:                 </div>
    30:                 <div class="saft_box col-content">
    31:                     <div class="saft_keyword">
    32:                         <strong><%= @saft.title.upcase %></strong>
    33:                     </div>
    34:                     <div class="saft_description">
    35:                         <p><%= @saft.description %></p>
  app/views/safts/show.html.erb:32:in `_app_views_safts_show_html_erb___758994895_2167416580'

How can I get this working?