List items from a collection by front matter array values - jekyll

Question

I have a Jekyll site with a collection, colleges, that I want to sort and display by a custom rankings front matter attribute. Here is an example of items in the collection:
---
title: Example College
score: 88
rankings:
- top accredited colleges
- most affordable
---
...
---
title: Sample College
score: 75
rankings:
- most affordable
- best online programs
---
...
---
title: Example University
score: 75
rankings:
- top accredited colleges
- best online programs
---
I would like to display the different rankings as headings with the colleges listed under them like so:
top accredited colleges
- Example College
- Example University
most affordable
- Example College
- Sample College
best online programs
- Sample College
- Example University
I've tried using Liquid's group_by filter like so:
{% assign groups = site.colleges | group_by: "rankings" %}
{% for group in groups %}
<h2>{{ group.name }}</h2>
<ul>
{% for college in group.items %}
<li>{{ college.title }}</li>
{% endfor %}
</ul>
{% endfor %}
But this seems to group the rankings front matter by an exact match of the full array rather than separating out the different values it contains:
<h2>[top accredited colleges, most affordable]</h2>
<ul>
<li>Example College</li>
</ul>
<h2>[most affordable, best online programs]</h2>
<ul>
<li>Sample College</li>
</ul>
<!-- etc -->
How do I separate out the individual values from the rankings front matter array in the output?

Answer 1

This can do the trick :
{% assign allRankings = site.colleges | map: "rankings" %}
{% assign rankingArray = "" | split:"" %}
{% for rankings in allRankings %}
{% assign rankingArray = rankingArray | concat: rankings | uniq %}
{% endfor %}
{% for ranking in rankingArray %}
<h2>{{ ranking | capitalize }}</h2>
<ul>
{% for college in site.colleges %}
{% if college.rankings contains ranking %}
<li>{{ college.title }}</li>
{% endif %}
{% endfor %}
</ul>
{% endfor %}