rails many to many association with foreign key
# text.rb
class Text < ActiveRecord::Base
  has_many :text_variables, :foreign_key => :text_sid, :primary_key => :sid
  has_many :variables, :through => :text_variables
end

# text_variable.rb
class TextVariable < ActiveRecord::Base
  belongs_to :text, :foreign_key => :text_sid
  belongs_to :variable
end

# variable.rb
class Variable < ActiveRecord::Base
  has_many :text_variables
  has_many :texts, :through => :text_variables
end

<!-- index.html -->
<% for text in @texts %>
  <h2><%= text.name %></h2>
  <div name="<%= text.sid %>">
    <%= form_for text, :url => "" do |f| %>
      <%= f.hidden_field :id, { :value => text.id } %>
      <% for vari in @varis %>
        <%= check_box_tag :variable_ids, vari.id, text.variables.include?(vari), :name => 'text[variable_ids][]', :style => 'width:auto' -%>
        <%= label_tag :variable_ids, vari.name -%><br />
      <% end %>
      <%= f.submit "Opslaan", :class => "submit", :style => "float:none;" %>
    <% end %>
  </div>
<% end %>