Sorting custom collections in Jekyll

jekyll_sort.rb

# Two files, default sort order is alphabetical ascending on file name.

# _config.yml

collections:
  albums:
    output: true

# _albums/audio_video_disco.md
---
title: Audio, Video, Disco
sortable: 2
---

# _albums/cross.md
---
title: Cross
sortable: 1
---

# index.md
---
---
<ul>
{% assign posts = site.albums | sort: "sortable" %}
 {% for post in posts %}
   <li>{{post.title}}</li>
 {% endfor %}
</ul>