Image urls retrieved using facebook graph api break after certain amount of time

I have a rails(ruby 2.2.1, rails 4.2.0) application that uses the koala gem (2.0.0) to retrieve all the images from all the albums for the user. The code for the facebook service is as

class FacebookService
 attr_reader :graph

 def initialize(token)
   @graph = Koala::Facebook::API.new(token)
 end

 def photos
   your_photos, photos_of_you = [], []

   albums_ids.map do |album_id|
     #this query will combine fetch all the photos in any album + comments + likes associated with that image
     your_photos += graph.get_connections(album_id, "photos", {limit: 100, fields: ["id", "source", "images", "height", "width", "created_time", "likes", "comments{id, message, from{name, picture}, created_time}"]})
end

     photos_of_you = graph.get_connections("me", "photos", {limit: 100, fields: ["id", "source", "images", "height", "width", "created_time", "likes", "comments{id, message, from{name, picture}, created_time}"]})

     (your_photos + photos_of_you).flatten
   end

   def albums_ids
     albums.map { |album| album["id"] }
   end

   def albums
     graph.get_connections("me", "albums")
   end
 end

and has been used as

fb = FacebookService.new current_user.oauth_token
photos = fb.photos

I am saving those urls in the db for further reference. I tried saving the source and the largest image from the images array but the urls keep going invalid. The problem is that when the above code is executed, the urls in the source and images aren't valid after certain amount of time(say 2-3 days). How can I retrieve image url that don't break later?

Thanks.

I filed a bug about this one a while back - it appears it's by design, and that there's some reason for those expiring URLs. The solution here is to actually cache image data, rather than just a URL.