Update REST API to handle slug refs

[getdave]

What?

Sub PR of #42809 containing changes to REST API needed to handle slugs as references for Navigation posts.

Why?

Foundational work for using slugs as references instead of IDs in the Nav block.

How?

Updates the REST API with a new endpoint which handles string-based slugs specifically and then routes that along the same handlers established for int based IDs.

Also adds test coverage for the endpoint.

Testing Instructions

• Check backwards compat - Nav block should still find menus by ID. Just create a Nav block with a menu and save then ensure that you can reload the page and the menu is found.
• Run the tests:

npm run test:unit:php /var/www/html/wp-content/plugins/gutenberg/phpunit/class-gutenberg-rest-navigation-controller-test.php

Screenshots or screencast

[getdave]

Both id and slug are needed in this request otherwise backwards compatibility is broken.

[spacedmonkey]

Wrong return type.

[spacedmonkey]

Why can you using existing endpoints, like this?

https://spacedmonkey.com/wp-json/wp/v2/posts?slug=proposal-add-a-dominant-color-background-to-images

[getdave]

Why can you using existing endpoints, like this?

https://spacedmonkey.com/wp-json/wp/v2/posts?slug=proposal-add-a-dominant-color-background-to-images

I feel like I tried that (it was a while ago now). I will double check that in the main PR just in case before we look to merge this one.

[spacedmonkey]

@getdave Works fine in my testing.

https://spacedmonkey.com/wp-json/wp/v2/navigation?slug=navigation-2

The same of the response is different. Any array with a single object over one object. But it works just fine.

[getdave]

The slug query returns a collection whereas an ID query returns a single post

https://spacedmonkey.com/wp-json/wp/v2/navigation/434

Gutenberg's Core data has:

• getEntityRecord - get a single post by recordKey. This is typically an ID but can be anything as long as the API endpoint accepts it (e.g. https://spacedmonkey.com/wp-json/wp/v2/navigation/{my-slug-here}).
• getEntityRecords - get a collection of posts. Can provide a query param to pass a query to the endpoint.

Conceptually what the block is trying to do is get the individual Navigation post assigned to it by a specific reference. This is different to fetching a collection of Navigation posts based on some query parameters.

This is why we went down the route of changing the endpoint to accept slug as the "reference". This also brings it more into line with the way the Template Parts endpoint works. For example - if you want a single template part you pass a slug as a reference as you get a single record back.

I'm diving back through the parent PR again because I"m sure I explored the route you are suggesting in more detail and it was decided against but I've not found the original conversation (yet).

[draganescu]

I think the way the REST API works is correct: the only call to return a single item is when we query by key. In general the key in WP is some kind of ID. The slug is a general query that can return a collection of one item if the slug is unique. With the caveat that I am not deep into the effort here yet, I think the client data library should work with this not the other way around.

[spacedmonkey]

The logic in javascript should be simple.

if is int
wp/v2/navigation/?include=234&per_page=1
else
wp/v2/navigation/?slug=slug&per_page=1

Then just check the response is empty and get the 0 value of response.

No need for a new endpoint.

[getdave]

I think the client data library should work with this not the other way around.

This needs to be explored in more detail but in principle I agree.

@spacedmonkey and I discussed this in DMs on WP Slack and we came to the conclusion that

1. I should (re)explore the idea of fetching the 0th item from a collection query.
2. Determine whether the Gutenberg Data Layer needs to adapt to handle this situation.

I'll report back with another PR.