class User
  has_many :posts, :include => :comments
  has_many :comments, :through => :posts
end

class Post
  belongs_to :user
  has_many :comments
end

class Comment
  belongs_to :post
  belongs_to :creator, :class_name => 'User', :foreign_key => 'creator_id'
end

class CommentsController
  def create
    # params[:comment] is { :text => 'Blah', :post_id => 1 }
    current_user.comments.create!(params[:comment])
    # render something
  end
end