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.
git-svn-id: https://develop.svn.wordpress.org/trunk@38667 602fd350-edb4-49c9-b593-d223f7449a82
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