Add relation parameter to REST API /wp/v2/posts endpoint

[earnjam]

The REST API /wp/v2/posts endpoint allows filtering by terms across multiple taxonomies, but it always uses an AND relationship between them.

Basically:
/wp/v2/posts?tags=1&categories=2

Translates to:
All posts with Tag 1 AND Category 2

WP_Query supports passing a relation parameter when running a tax query across multiple taxonomies. It defaults to AND, which is what we do now, but it would be nice to support the ability to specify OR here as well.

[earnjam]

44326.diff​:

• Adds a new parameter to the /wp/v2/posts endpoint called relation which accepts AND or OR. If left off, it defaults to the current functionality of AND.
• Adds 2 new tests

[TimothyBlynJacobs]

This looks good to me!

I think we just need a different query parameter name that is a bit more specific to the term query. Perhaps terms_relation?

[earnjam]

Thanks! I chose relation since that was the name of the parameter in WP_Query, though I can see how that is pretty vague. I like term_relation, but I wonder if it should be something like tax_relation since it's the relationship between the taxonomies, not the individual terms.

This ticket actually spun out of #41287, which is for supporting the use an AND relationship between terms within a single taxonomy. I think the two would pair up nicely to extend what is possible through the API.

[TimothyBlynJacobs]

tax_relation sounds good to me!

[earnjam]

44326.2.diff​ just updates the naming to tax_relation

[TimothyBlynJacobs]

In 46646:

REST API: Add tax relation parameter to posts collection.

The REST API supports filtering by terms across multiple taxonomies using an AND relation. This adds support for an OR relation by adding "tax_relation=OR" as a query parameter.

Props earnjam.
Fixes #44326.

[SergeyBiryukov]

In 46648:

REST API: Set orderby parameter in tax relation unit tests added in [46646].

The default value of date caused a race condition resulting in random test failures.

See #44326.

[desrosj]

This was detailed in the following dev note: ​https://make.wordpress.org/core/2020/02/29/rest-api-changes-in-5-4/