drz_help1.rb

class Post
  has_many :cat_relations
  has_many :categories, through: :cat_relations
end

class Category
  has_many :cat_relations
  has_many :posts, through: :cat_relations
end

class CatRelation
  belongs_to :post
  belongs_to :category
end

# ...
p = Post.first
p.categories # This returns the Category objects associated with the Post