post.rb

class Post < ActiveRecord::Base
  scope :task -> { joins(:ideas)}
  scope :crowdfunding -> { joins(:crowdfunding)}
  scope :tailored_for_user -> { order("created_at DESC").tagged_with.current_user.tag_list }
  scope :popular -> { order("cached_votes_score DESC") }
...
end

class PostsController < ApplicationController
  ...
  def index
    @posts = Post[params.filter][params.sort]
  end
end