How do I define a custom collection in Jekyll? - jekyll

Question

Following the Jekyll Collections documentation, I wrote the following code in _config.yml
_config.yml
collections:
- popular_posts
So when I print {{ site.collections }}, the output is "popular_posts".
I also made a folder called "_popular_posts" at the same level as "_posts".
_popular_posts contains two .md files with some YAML front matter, same as a post.
However, if I print {{ site.popular_posts }} or {{ site.collections.popular_posts }}, there is no output.
How do I have Jekyll recognize the .md files in that directory so that the following code will work?
{% for popular_post in site.popular_posts %}
<a href="{{ popular_post.link }}">
<h1>{{ popular_post.title }}</h1>
<img class="pop-img" src="{{ popular_post.image_url }}">
</a>
<span id="pop-order"><span class="pop-current-popular_post-number">{{ popular_post.number }}</span>/5</span>
{% endfor %}

Answer 1

It's quite easy! You're on the right track. In your _config.yml:
collections:
- popular_posts
This will tell Jekyll to read in all the files in _popular_posts.
If you want each of those two files to have a corresponding output file (like how _posts works now), you'll want to change your _config.yml to:
collections:
popular_posts:
output: true
This will produce files at /popular_posts/filename1.html and /popular_posts/filename2.html, one page for each post.
Collections are only recently up on GitHub Pages so if you were trying this there, it would have failed.
Check out jekyll-help for more help if you need it!