post.rb

class Post < ActiveRecord::Base
 
  attr_accessor :position, :follower_id
  
  has_many :relationships, :dependent => :destroy,
                               :foreign_key => "follower_id"

  has_many :reverse_relationships, :dependent => :destroy,
                                      :foreign_key => "followed_id",
                                      :class_name => "Relationship"

  has_many :followers, :through => :reverse_relationships,
                                   :source  => :follower
  
  accepts_nested_attributes_for :relationships
  
end

relationship.rb

class Relationship < ActiveRecord::Base
  attr_accessible :followed_id, :latitude

  belongs_to :follower, :class_name => "User"
  belongs_to :followed, :class_name => "Post"
    
  validates :follower_id, :presence => true
  validates :followed_id, :presence => true
  
end

relationships_controller.rb

class RelationshipsController < ApplicationController

  def sort
    params[:post].each_with_index do |id, index|
      Relationship.update_all({position: index+1}, {id: id})
    end
    render nothing: true
  end
  
end