Comments: Allow WP_Comment_Query fields to project arbitrary columns

[dd32]

Summary

• Extends WP_Comment_Query's fields argument to accept any column of $wpdb->comments (single string) or an array of columns, mirroring the flexibility of WP_User_Query.
• Projections apply DISTINCT so the common array_unique( wp_list_pluck( ... ) ) pattern collapses to a single 'fields' => 'comment_post_ID' query.
• 'ids' and '' (default) keep their existing semantics — the change is purely additive.

Implements Trac #65313.

Why

Plugins that want "give me the distinct comment_post_IDs matching X" currently have three options, none good:

1. fields => '' + array_column( $q->comments, 'comment_post_ID' ) — hydrates every comment object just to throw all but one field away.
2. fields => 'ids' + get_comment( $id )->comment_post_ID in a loop — N+1 cache lookups.
3. Drop to raw SQL — bypasses filters, caching, and meta-query.

In the gp-translation-helpers reference case, hydrating ~32K comments to extract distinct post IDs took ~6.6s; the equivalent raw SQL returned the same 25,694 IDs in ~610ms (~11× speedup, all of it from skipping hydration).

API

// Returns an array of distinct post IDs (no array_unique needed):
$post_ids = ( new WP_Comment_Query() )->query( array(
    'meta_key'   => 'locale',
    'meta_value' => 'nl',
    'user_id'    => 42,
    'fields'     => 'comment_post_ID',
) );

// Multi-column projection returns an array of stdClass rows:
$rows = ( new WP_Comment_Query() )->query( array(
    'fields' => array( 'comment_ID', 'comment_post_ID', 'comment_date' ),
) );

Column names must be passed in exact case. The only exception is the ID segment of comment_ID / comment_post_ID, which accepts any case (comment_id, comment_Id, comment_ID).

Unknown column names fall through silently to the default behaviour (WP_Comment objects), to keep the path forward-compatible.

Test plan

• npm run test:php -- tests/phpunit/tests/comment/query.php — 165 tests / 487 assertions pass (no regressions).
• npm run test:php -- --group comment — 539 tests / 1365 assertions pass.
• New tests in tests/phpunit/tests/comment/query.php cover:
  • Single-column projection returns distinct values
  • Single-column projection with a custom column + filters
  • Case-flexibility of the ID suffix; strict case for non-ID columns
  • Multi-column projection returns stdClass[] with DISTINCT applied
  • Unknown column falls back to full WP_Comment objects
  • 'ids' semantics unchanged (still returns int[])
  • Empty result sets return array() without a follow-up query

🤖 Generated with Claude Code

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[dd32]

Re-filing on WordPress/wordpress-develop per ticket workflow. See follow-up PR.