I got an uninitialized constant error, help!!

hey guys, i am new to rails, so please be nice to me if i ask stupid
questions.

I ran into this problem as I try to play with many to many
association.

I have three objects,

Category
  has_many :artist, :through => artist_category
  has_many :artist_category
Artist
  has_many :category, :through =>artist_category
  hash_many :artist_category
  attr_accessor :category_ids

  after_save :update_category

  def update_category
     unless category_ids.null?
       self.artist_category.each do |m|
          m.destory unless category_ids.include?(m.category_id.to_s)
          category_ids.delete(m.category_id.to_s)
        end

        category_ids.each do |g|
          self.category_artist.create(:category_id => g) unless
g.blank?
         end
         reload
      end
    end

Artist_Category
  belongs_to: category
  belongs_to: artist

============ on the artists_controller, i have new and create method

You're mixing and matching :artist, :artists, Artists, Artist. Your
has_manys should be :artists and when you're talking about the class,
then it should always be Artist (which should be in artist.rb)

Fred