Group collection items by year on a page

I’m trying to create a page that organizes the content of my website by year. My website is using collections, rather than posts, and I’m having a difficult time creating a page that groups all collection items by year.

Here’s where I’m at:

{% assign research = site.documents | where:"research","true" %}
	{% assign postsByYear = research | group_by_exp:"post", " | date: '%Y'" %}
	{% for year in postsByYear %}
	  <h3 id="{{ | slugify }}">{{ }}</h3>
		{% for post in year.items %}
		{% assign postYear = | date:"%Y" %}
				<h4><a href="{{ post.url }}">{{ post.title }}</a></h4>
				<h6>{{ }}</h6>
				<div>{{ post.abstract | truncatewords: 75 }} </div>
			{% endfor %}
	{% endfor %}

This returns posts from the past three years in an odd order: 2017, 2018, 2016. I can’t figure out why it’s not starting with the current year.

Any ideas?

Collections aren’t required to have the date in them like posts - so are you adding a front matter field for the date?

Have you tried the sort filter? or maybe some dates aren’t formatted the same as others?

hey thank you so much for this! it worked perfectly!