[wp-trac] [WordPress Trac] #45107: Taxonomies should only be allowed to support one object type

[WordPress Trac]

#45107: Taxonomies should only be allowed to support one object type
-------------------------+-------------------------------------------------
 Reporter:  desrosj      |      Owner:  (none)
     Type:  enhancement  |     Status:  new
 Priority:  normal       |  Milestone:  Awaiting Review
Component:  Taxonomy     |    Version:
 Severity:  normal       |   Keywords:  2nd-opinion needs-patch needs-unit-
  Focuses:               |  tests
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 Currently, taxonomies can be registered to any object type (posts,
 comments, users, etc.). But core does not enforce a one to one limit for
 object types to taxonomies, which can be problematic.

 For example, if a taxonomy is registered to both users and posts, there
 can be unintended consequences. Adding a term to a post with an ID of 3
 would also cause a user with an ID of 3 to have that term. Removing that
 term from the user would also affect the post. Unique IDs are only
 enforced on a per object type basis, not accross all types.

 The approach here would be to introduce a `_doing_it_wrong()` notice (and
 possibly even return a `WP_Error`) when a taxonomy is registered to
 multiple object types.

 **Good:** `register_taxonomy( 'custom_tax_name', array( 'post', 'page',
 'cpt' ) );`

 **Bad:** `register_taxonomy( 'custom_tax_name', array( 'post', 'user' )
 );`

 == Why ==

 Adding this to Core would open the door for the following potential
 features:

 - `WP_Tax_Query` support could be added to users (see #31383), comments,
 etc.
 - Built-in fields for taxonomy could be added to the REST API for users,
 comments, etc.
 - UIs could be added for users (also see #31383), comments, etc.

 == Backward Compatibility ==

 To continue supporting backward compatibility for sites that are
 registering a taxonomy for multiple object types, `register_taxonomy()`
 could continue working as is. The only change would be to return a
 `WP_Error` and a `_doing_it_wrong()` notice.

 In the future, `register_taxonomy()` could be changed to only register
 objects with the same type as the first specified object type.

 Example: `register_taxonomy( 'custom_tax_name', array( 'post', 'user',
 'page' ) );` would only register the taxonomy for posts and pages (same
 object type).

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Ticket URL: <https://core.trac.wordpress.org/ticket/45107>
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