Filtering content based on first parameter in array in Jekyll

Question

In my posts' Front Matter, I have "categories" which is an array.

I'm looking for a way to filter the posts based on the first element in the categories array.

For example, if I had two posts' Front Matter like:

title: Post Number One 
categories:
   - first post ever
   - cool stories

and

title: Post Two
categories:
- cool stories

I want a way to filter on categories where "cool stories" would return only "Post Two" because "cool stories" shows up as the first element of the array.

Answer 1

There are several ways to implement this feature. One of which is:

Create a new include file in _includes named first-category.html with the following code:

{% assign chosen_category = include.category %}

{% for post in site.posts %}
{% for category in post.categories.first %}
{% if category == chosen_category %}
  {{ post.title }}
{% endif %}
{% endfor %}
{% endfor %}

Then, in the page where you're listing the post which have the first category as the one in question, simply include the above file and pass the chosen category name:

## Post that have the first category of "cool stories"

{% include first-category.html category = "cool stories" %}

## End

The above code will only show posts which have "cool stories" as the first category in the posts' front-matter.

Answer 2

This is an Information Architecture (IA) question.

• any post must be categorized in a main category
• post can be categorized in more than one " categorie "

Let's use Jekyll's category/categories inner working to represent our IA.

If you define a post like this :

---
title:  "My post"
category: "main category"
categories:
  - other
  - wat!
# ... more front matter variables
---

Category/categories will be available as :

post.category => main category
post.categories =>
  - other
  - wat!
  - main category

Now if you want to use category to filter your posts, using group_by and where_exp filters , you can do :

{% assign category = "main category" %}

{% comment %} #### Grouping posts by 'main' category {% endcomment %}
{% assign grouped = site.posts | group_by: "category" %}
{{ grouped | inspect }}

{% comment %}#### Get our category group{% endcomment %}
{% assign categoryPosts = grouped | where_exp: "group", "group.name == category" | first %}
{{ categoryPosts | inspect }}

{% comment %} #### All interesting posts are now in categoryPosts.items {% endcomment %}
{{ categoryPosts.items | inspect }}

{% comment %} #### We can now sort and loop over our posts {% endcomment %}

{% assign sorted = categoryPosts.items | sort: "whateverKeyYouWantToSortOn" %}
<ul>
{% for post in sorted %}
 <li>
  <a href="{{ site.baseurl }}{{ post.url }}">{{ post.title }}</a>
  <br>Category : {{ post.category }}
  <br>Categories :
  <br><ul>{% for c in post.categories %}
   <li>'categorie' {{ forloop.index }} - {{ c }}</li>
  {% endfor %}</ul>
 </li>{% endfor %}
</ul>