Filter site.related_posts in Jekyll - ruby

Question

I am very new to Jekyll and Ruby (yet, very excited).
Without using a plugin, I am trying to find a way to filter the site.related_posts.
For example, I am reading the post with title Foo and categories A, B.
The site contains in total 3 posts:
Foo (Categories: A, B)
Bar (Categories: A, C, D)
Zoo (Categories: B, F)
By the default, in Jekyll we do this:
{% for post in site.related_posts limit:5 %}
{% endfor %}
However, the above code returns all the (3) posts.
A post contains many categories, so categories should be an array.
How can I modify the code and return only those whose categories intersect with the current post's categories?
(In this example, I would like the code to return only Foo and Zoo.)

Answer 1

I don't have the ability to test this right now, but something like this will work given Liquid's limited syntax:
{% for post in site.related_posts limit:5 %}
{% assign match = false %}
{% for category in post.categories %}
{% if page.categories contains category %}
{% assign match = true %}
{% endif %}
{% endfor %}
{% if match %}
<li>{{ post.title }}</li>
{% endif %}
{% endfor %}

Answer 2

Make sure each post has a category in the YAML front matter, then add this to where you would like to show the post relating CATEGORY_NAME:
{% for post in site.categories.CATEGORY_NAME %}
<li>
<a href="{{ post.url }}">
<img src="{{ post.thumbnail }}">
<p>{{ post.excerpt }}</p>
</a>
</li>
{% endfor %}