The new 'object_ids' parameter for `WP_Term_Query` allows queries for
terms that "belong to" a given object. This change makes it possible
to use `WP_Term_Query` inside of `wp_get_object_terms()`, rather than
assembling a SQL query.
The refactor has a couple of benefits:
* Less redundancy.
* Better consistency in accepted arguments between the term query functions. See #31105.
* Less redundancy.
* Object term queries are now cached. The `get_object_term_cache()` cache remains, and will be a somewhat less fragile secondary cache in front of the query cache (which is subject to frequent invalidation).
* Less redundancy.
A small breaking change: Previously, if a non-hierarchical taxonomy had
terms that had a non-zero 'parent' (perhaps because of a direct SQL
query), `wp_get_object_terms()` would respect the 'parent' argument.
This is in contrast to `WP_Term_Query` and `get_terms()`, which have
always rejected 'parent' queries for non-hierarchical taxonomies. For
consistency, the behavior of `get_terms()` is being applied across the
board: passing 'parent' for a non-hierarchical taxonomy will result in
an empty result set (since the cached taxonomy hierarchy will be empty).
Props flixos90, boonebgorges.
See #37198.
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Prior to the introduction of `WP_Term_Query`, the 'orderby' clause
passed to the 'terms_clauses' filter was prefixed by `ORDER BY`. After
`WP_Term_Query`, this was not the case; `ORDER BY` was added after the
filter. As such, plugins filtering 'terms_clauses' and returning an
'orderby' clause beginning with `ORDER BY` resulted in invalid syntax
when `WP_Term_Query` prepended a second `ORDER BY` keyword to
the clause.
This changeset rearranges the way the 'orderby' clause is built so that
it will be passed to 'terms_clauses' in the previous format.
Fixes#37378.
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`WP_Term_Query` is modeled on existing query classes, such as those used
for comments and users. It provides a more consistent structure for generating
term queries, and should make it easier to add new functionality in the future.
Props flixos90, boonebgorges.
See #35381.
Built from https://develop.svn.wordpress.org/trunk@37572
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