Rails/Dragonfly: is it possible to decode URL (/media/W1siZiIsInRzYW...) to get the model instance it refers to?

This is what I want to accomplish:

On an extranet with a feed wall, users just type non-formatted text (non style, just \n and links recognition).

But quite often, users want to add a link to a document which is stored in the same extranet (using dragonfly). Obviously, the link is quite awful to display (ex: https://extranet.com/media/l0ngUiD/original_filename.pdf?sha=31310881DAEF1). This document refers to a Document instance which has a nice title, ex: "Original Filename (PDF)" I would like those links to be (automatically) replaced by

<a href="https://extranet.com/media/l0ngUiD/original_filename.pdf?sha=31310881DAEF1">Original Filename (PDF)</a>

Problem is: how to find which model and which document this document refers to, using the UID and sha.

I guess this is possible as Dragonfly decodes the url, but I can't find how to (not much comments in the code).

Solution

The variant's UID is Base64 encoded, so when decoding the UID you will get a JSON encoded array of Dragonfly processor steps.

Here an actual example from a live database:

uid = 'W1siZiIsIjIwMTUvMDcvMDkvMDkvMTMvMDIvOTE3LzE5NDg3ODk0MDc1XzE4NGYzMjc0MWVfay5qcGciXV0'
Base64.decode64 uid
# => "[[\"f\",\"2015/07/09/09/13/02/917/19487894075_184f32741e_k.jpg\"]]"

Each step is an array with the step's operation in the first element and the step's arguments in the remaining.