Source code for: http://baylorrae.com/using-ransack-with-habtm/

models.rb

# product.rb
class Product < ActiveRecord::Base
  has_many :categorizations
  has_many :categories, :through => :categorizations
end

# category.rb
class Category < ActiveRecord::Base
  has_many :categorizations
  has_many :products, :through => :categorizations
end

# categorization.rb
class Categorization < ActiveRecord::Base
  belongs_to :category
  belongs_to :product
end

ransack_search_form.html.erb

<%= search_form_for @query do |f| %>
  <%= f.label :categories_id_eq, "Category" %>
  <%= f.collection_select :categories_id_eq, Category.order(:title), :id, :title %>

  <%= f.submit "Filter Products" %>
<% end %>