Creating an RSS feed for a single Jekyll Collection

Creating a generic rss feed template, specific to the current Jekyll collection.

My jekyll collection, jekyll_notes, is specified in _config.yml, and all the collection-related posts and pages will be in the folder _jekyll_notes:

collections:
...
  jekyll_notes:
    output: true
    title: Jekyll Notes
    title_singular: A Jekyll Note
    description: a collection of notes and observations from and aide-memoires for using Jekyll for the first time.
    sequence: 100
...

Create a file called _jekyll_notes/feed.xml

---
layout: null
---
{% include collection_feed.xml %}

and create the file, _includes/collection_feed.xml

<?xml version="1.0" encoding="UTF-8"?>
{% assign collection_name = page.collection %}{%
assign collection = site.collections | where:"label", collection_name | first %}{%
assign posts = site[collection_name] | where:"layout", "post" | sort: 'date')
%}<rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom" xml:lang="{{ site.lang }}">
    <channel>
        <title>{{ collection.title }}</title>
        <link>{{ site.url }}</link>
        <copyright>All rights reserved.</copyright>
        <description>{{ collection.description }}</description>
        <atom:link href="{{ page.url | absolute_url }}" rel="self" type="application/rss+xml" />
        {% for post in posts limit:20 %}
            <item>
                <title>{{ post.title | xml_escape }}</title>
                <description>{{ post.excerpt | xml_escape }}</description>
                <pubDate>{{ post.date | date_to_rfc822 }}</pubDate>
                <link>{{ post.url | absolute_url }}</link>
                <guid isPermaLink="true">{{ post.url | absolute_url }}</guid>
            </item>
        {% endfor %}
    </channel>
</rss>

The handlebars code

obtains the collection of the current page (which is available via the param page, automatically passed through via the {% include collection_feed.xml %} in the feed.xml file)
looks up the full details of the collection,
gets the list of all documents in that collection
- which are of layout: post,
sorts them by date,
and then constructs the RSS feed of the most recent ones.

spurious newlines breaks the XML spec

NB, because the handlebar processing has side-effects and leaves newlines in the rendered file, naively writing the handlebar code as

{% assign collection_name = page.collection %}
{% assign collection = site.collections | where:"label", collection_name | first %}
{% assign posts = site[collection_name] | where:"layout", "post" | sort: 'date' %}

… will create an invalid RSS file, depending where in the xml the linefeeds occur.

Instead, doing the least-worst mangling of the code as follows

<?xml version="1.0" encoding="UTF-8"?>
{% assign collection_name = page.collection %}{%
assign collection = site.collections | where:"label", collection_name | first %}{%
assign posts = site[collection_name] | where:"layout", "post" | sort: 'date')
%}<rss version="2.0"

… leaves it more or less as easy to read, but importantly not dropping spurious newlines into the XML output.