application_controller.rb

ApplicationController < ActionController::Base

  # Fetch the best location available to the app at the time
  def fetch_location
    if params[:location].present? # Geocode the manual location input
      session[:manual_location] = Geokit::Geocoders::MultiGeocoder.geocode( params[:location] )
    end

    if session[:manual_location].present?
      session[:manual_location]
    elsif current_user && current_user.city.present? # Use the user profile
      current_user
    else # Fallback to IP location
      session[:geo_location] ||= Geokit::Geocoders::MultiGeocoder.geocode(
        if Rails.env.production? || Rails.env.staging?
          request.remote_ip
        else # Development environments
          '217.43.175.61'
        end
      )
    end
  end
  
end

post.rb

class Post < ActiveRecord::Base
  DISTANCE_SCALE = 20.0
  
  acts_as_mappable
  
  scope :by_time, order( 'created_at DESC' )
  
  scope :by_distance, lambda { |origin|
    geo_scope( :origin => origin ).order(
      'CASE WHEN distance IS null THEN 1 ELSE 0 END, distance'
    )
  }
  
  scope :by_distance_time, lambda{ |origin, datetime|
    geo_scope( :origin => origin ).order(
      "( CASE WHEN distance IS null THEN distance ELSE 9999 END * ? ) * ABS(
        DATEDIFF( posts.created_at, ? )
      ) DESC",
      DISTANCE_SCALE,
      datetime
    )
  }
end

posts_controller.rb

class PostsController < ApplicationController

  def index
    @posts = Post.by_time_distance( fetch_location, Time.now )
    render :json => @posts
  end
  
end