How to save extra field values using has_may :through - ruby-on-rails-3

Question

I have these models
class Product < ActiveRecord::Base
# fields: name, weight, expiry_date, active...
has_many :categorizations
has_many :categories, :through => :categorizations
end
class Category < ActiveRecord::Base
# fields: name, active...
has_many :categorizations
has_many :products, :through => :categorizations
end
class Categorization < ActiveRecord::Base
# fields: user_id, type, priority, product_id, category_id...
belongs_to :product
belongs_to :category
validates :user_id, :presence => { :message => "Must be filled" }
end
_form.html.erb
<%= form_for #product do |f| %>
<%= f.error_messages %>
<%= f.text_field :name %>
<%= f.select :category_ids .... {:multiple => true} %>
<% end %>
parameters:
{"product"=>{"name"=>"sdf", "category_ids" => ["1", "2"]}, "commit"=>"Create Product"}
Question: How to save the some additional fields like user_id, priority in categoriztion table?
I tried to access categorization object after initialize product but there is empty array. for example
#product = Product.new(params[:product]) # {"product"=>{"name"=>"sdf", "category_ids" => ["1", "2"]}, "commit"=>"Update Document"}
#product.categorizations => []