Users: Separate WP_User_Query cache entries by result format

[akash-aman]

WP_User_Query fetches results in two different formats depending on fields: an array of fields returns rows as objects (get_results()), anything else returns a flat list of values (get_col()). A single-field array such as array( 'ID' ) generates exactly the same SQL as 'ID' or 'all', and generate_cache_key() builds the key from the SQL only. So whichever query runs first decides the format that every later query receives from the cache.

With a persistent object cache, this causes:

• objects returned where strings are expected, and vice versa;
• WP_User objects with an empty user_login, user_email, etc. from get_users() after get_users( array( 'fields' => array( 'ID' ) ) );
• a fatal error for 'all_with_meta' (Cannot access offset of type stdClass on array in class-wp-user-query.php), e.g. in wp user list, Users → All Users, and on multisite Network Admin → Users.

Full reproduction details are in comment:3 on the ticket.

Changes

src/wp-includes/class-wp-user-query.php: generate_cache_key() now includes the result format in the key:

$format = is_array( $this->query_vars['fields'] ) ? 'objects' : 'values';

$key = md5( $sql );

return "get_users:$format:$key";

• Readable keys: array queries use get_users:objects:<hash>, and all other queries use get_users:values:<hash>.
• Same hit rate: 'ID', 'all' and 'all_with_meta' still share an entry, because they cache the same list of IDs.
• Upgrade-safe: every key changes, so entries cached incorrectly before this change are not read afterwards.

tests/phpunit/tests/user/queryCache.php: adds test_query_cache_should_respect_fields_return_format() with a data provider of 6 cases:

Case	First query (primes the cache)	Second query (read from the cache)
string then array	'ID'	array( 'ID' )
array then string	array( 'ID' )	'ID'
array then all	array( 'ID' )	'all'
all then array	'all'	array( 'ID' )
array then all_with_meta	array( 'ID' )	'all_with_meta'
array then string field	array( 'user_email' )	'user_email'

Each case compares the cached result with the same query run with 'cache_results' => false. It checks the format of every item with assertSame(), then the values.

Testing

Tested on trunk with Redis as the persistent object cache:

• New test: 6/6 pass with the fix. Without it: 4 failures ("Cached results should have the same format as uncached results.") and 2 errors from the 'all' / 'all_with_meta' cases.
• phpunit --filter Tests_User_Query_Cache: 42 tests, all pass.
• phpunit --group user: 1350 tests, no failures.
• PHPCS: no errors.
• Manual: the steps in the ticket description, wp user list, the Users list table, and Network Admin → Users on a multisite network all work. Entries cached incorrectly before the change are not returned after it.

Related

#13727 by @irozum takes the same approach, with a hash-based key and a single-case test. This PR differs in its readable objects / values key and its broader test coverage. Happy to consolidate into one PR, or to have the data provider added to #13727, whichever reviewers prefer.

Trac ticket: https://core.trac.wordpress.org/ticket/62003

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5.5
Used for: Investigating the root cause, reproduction scripts, implementation, tests and this description. I reproduced the issue myself on local sites, and I reviewed and revised the implementation and tests.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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