Add Link: rel="alternate" header pointing to JSON for a table/query

[simonw]

Originally explored in simonw/datasette-notebook#2 (comment) - I wanted an efficient way to scan a list of URLs and figure out which if any of those corresponded to Datasette tables, canned queries or SQL output that could be represented as a table on a page.

It looks like a neat way to do that is with Link: header like this:

Link: http://127.0.0.1:8058/fixtures/compound_three_primary_keys.json; rel="alternate"; type="application/datasette+json"

I can put a <link href=... in the page header too.

[simonw]

I built an initial prototype of this in a branch: e0a8469 - which exposed even more flaws in the way TableView is structured (adding custom HTTP headers to the response is way harder than it should be) which I should address in the refactor in #617.

[simonw]

I'm going to apply the hack, then fix it again in:

• Complete refactor of TableView and table.html template #1518

[simonw]

Documentation: https://docs.datasette.io/en/latest/json_api.html#discovering-the-json-for-a-page

https://docs.datasette.io/en/latest/json_api.html top --cors section mentions the new Access-Control-Expose-Headers: Link header.

[simonw]

Demos - these pages both have <link rel=... if you view source on them:

• https://latest.datasette.io/fixtures/sortable
• https://latest.datasette.io/fixtures/sortable/a,a

And you can hit them with curl like so:

% curl -I 'https://latest.datasette.io/fixtures/sortable'    
HTTP/1.1 200 OK
link: https://latest.datasette.io/fixtures/sortable.json; rel="alternate"; type="application/json+datasette"
cache-control: max-age=5
referrer-policy: no-referrer
access-control-allow-origin: *
access-control-allow-headers: Authorization
access-control-expose-headers: Link
content-type: text/html; charset=utf-8
x-databases: _memory, _internal, fixtures, extra_database
Date: Wed, 02 Feb 2022 07:56:17 GMT
Server: Google Frontend
Transfer-Encoding: chunked

% curl -I 'https://latest.datasette.io/fixtures/sortable/a,a'
HTTP/1.1 200 OK
link: https://latest.datasette.io/fixtures/sortable/a,a.json; rel="alternate"; type="application/json+datasette"
cache-control: max-age=5
referrer-policy: no-referrer
access-control-allow-origin: *
access-control-allow-headers: Authorization
access-control-expose-headers: Link
content-type: text/html; charset=utf-8
x-databases: _memory, _internal, fixtures, extra_database
Date: Wed, 02 Feb 2022 07:56:24 GMT
Server: Google Frontend
Transfer-Encoding: chunked