Hierarchical custom data

[asbjornu]

I have a _data/publishers.js file that exports an async function which fetches data from an API:

// _data/publishers.js
module.exports = async function() {
  const url = "…"; 
  const response = await EleventyFetch(url, { duration: '1h', type: 'json' });
  return response.publishers;
}

I then have a publishers/publishers.liquid file used to render this data:

---
pagination:
    data: publishers
    size: 1
    alias: publisher
permalink: /{{ publisher.name | slugify }}/
eleventyComputed:
    title: '{{ publisher.name }}'
    description: All of {{ publisher.name }}'s books.
---

<h1>{{ title }}</h1>

<p>{{ description }}</p>

<ul class="books">
    {% for book in publisher.books %}
        <li>
            <a href="/{{ publisher.name | slugify }}/books/{{ book.id }}">{{ book.title }}</a>
        </li>
    {% endfor %}
</ul>

So far, everything works. The problem is arises when I want to add a book.liquid template for the URL /{{ publisher.name | slugify }}/books/{{ book.id }}. I've tried to add a _data/book.js file and a book/book.liquid template, but then I get the following error:

[11ty] Output conflict: multiple input files are writing to `_site/undefined/books/index.html`. Use distinct `permalink` values to resolve this conflict.

This would also not be an optimal solution, as _data/book.js is redundant. All the data required to render book.liquid already exists in the data returned by publisher.js. Does anyone have an idea how to solve this predicament in Eleventy?