问无法关联类别和帖子

EN

class Post < ActiveRecord::Base
  attr_accessible :body, :publish_date, :title, :user_id
  validates_presence_of :title, :body, :publish_date, :user_id
  belongs_to :user
  has_and_belongs_to_many :categories
end

class Category < ActiveRecord::Base
  attr_accessible :name
  validates_presence_of :name
  has_and_belongs_to_many :posts
end

<%  f.association :categories, :as => :checkboxes %>

undefined method `association' for #<ActionView::Helpers::FormBuilder:0xb5e82b40>