About a year ago I wrote up a blog post demonstrating how to set up an Eleventy blog that supports multiple authors (Supporting Multiple Authors in an Eleventy Blog). While you should definitely read that post first, the gist of it goes like this:

• Define authors in a simple JSON data file and ensuring each one has a unique key.
• Include an authors field in front matter so you can specify the author
• Define a filter that let you get the author's name for the post
• Define an author template that shows all the posts for the author

While this worked fine, a few days ago I got an email asking if this supported multiple authors per post. Unfortunately, it didn't. Despite me using authors as a front matter key name, if you actually did pass multiple authors it didn't recognize them as a list. I whipped up a quick modification to make this work.

First off, I decided on a simple comma-separated list for multiple authors. Here's an example:

---
title: Epsilon Post
layout: post
date: 2020-08-02
tags: post
author: brinaldi,rcamden
---

This is Epsilon

Note that YAML does support specifying an array, or list, but honestly, I find that format not terribly easy to use. (Ok, I'd have to Google it, so it's not that hard.)

After creating a post like this, I had to make two changes. First, consider the previous version of post.md:

---
layout: main
---

<h2>{{ title }}</h2>
{% if author %}
	{% assign myAuthor = authors | getAuthor: author %}

	<p>
	Written by <a href="/authors/{{ author }}">{{ myAuthor.name }}</a>
	</p>
{% endif %}

{{ content }}

It makes use of the getAuthor filter to convert the author primary identifier to a real name. To support multiple authors. I first change the getAuthor filter to getAuthors:

eleventyConfig.addFilter("getAuthors", (authors,label) => {
	let labels = label.split(',');
	return authors.filter(a => labels.includes(a.key));
});

All I do here is take the label, split it on commas, an filter to any author in the new array. Then, back in the post.md, I change it to this:

---
layout: main
---

<h2>{{ title }}</h2>
{% if author %}
	{% assign myAuthors = authors | getAuthors: author %}
	<p>
	Written by 
		{% for theAuthor in myAuthors %}
			<a href="/authors/{{ theAuthor.key }}">{{ theAuthor.name }}</a>{% unless forloop.last %}, {% endunless %}
		{% endfor %}
	</p>
{% endif %}

{{ content }}

You can see now that I loop over a list of authors and output a comma if the loop has more than one item. This made individual blog posts work fine. Here's a post with two authors:

The second change I had to make was to the authors template. The template was fine, but the getPostsByAuthor filter had to be updated:

eleventyConfig.addFilter("getPostsByAuthor", (posts,author) => {
	return posts.filter(p => {
		if(!p.data.author) return false;
		let authors = p.data.author.split(',');
		return authors.includes(author);
	});
});

And that's basically it. I did not edit the previous demo in my GitHub repository as I wanted a nice way to compare both, so instead I saved this into a new directory: https://github.com/cfjedimaster/eleventy-demos/tree/master/multiauthor2

As always, if you've got any questions about this, just let me know!