BaseRelationField queryCondition and ElementQuery's siteId

CHANGELOG-WIP.md

@@ -58,6 +58,7 @@
 - Added `craft\base\ElementInterface::deletionBlockers()`. ([#18728](https://github.com/craftcms/cms/pull/18728))
 - Added `craft\base\ElementInterface::setDirtyFieldTracking()`.
 - Added `craft\elements\PopulateElementEvent::$content`.
+- Added `craft\elements\db\ElementQuery::$activeQuery`.
 - Added `craft\elements\db\ElementQueryInterface::collectIds()`.
 - Added `craft\elements\deletionblockers\BaseDeletionBlocker`. ([#18728](https://github.com/craftcms/cms/pull/18728))
 - Added `craft\elements\deletionblockers\DeletionBlockerInterface`. ([#18728](https://github.com/craftcms/cms/pull/18728))
@@ -94,5 +95,6 @@
 - Updated the built-in composer.phar to 2.9.7. ([#18761](https://github.com/craftcms/cms/issues/18761))
 - Fixed a bug where nested entries weren’t getting loaded with their content, if they had an entry type that was no longer allowed by their Matrix field.
 - Fixed the wording of the validation error when saving a nested entry with an invalid entry type. ([#18506](https://github.com/craftcms/cms/issues/18506))
+- Fixed a bug where relation fields’ element query params weren’t limiting results based on the query’s target site(s). ([#18781](https://github.com/craftcms/cms/issues/18781))
 - Fixed a [high-severity](https://github.com/craftcms/cms/security/policy#severity--remediation) potential RCE vulnerability. (GHSA-f74w-488g-8x5r)
 - Fixed a [moderate-severity](https://github.com/craftcms/cms/security/policy#severity--remediation) JavaScript injection vulnerability. (GHSA-c55v-343g-5xff)

src/elements/db/ElementQuery.php

@@ -111,6 +111,13 @@ class ElementQuery extends Query implements ElementQueryInterface
      */
     public const EVENT_AFTER_POPULATE_ELEMENTS = 'afterPopulateElements';
 
+    /**
+     * The current element query instance being prepared, for reference by fields’ `queryCondition()` methods.
+     *
+     * @since 5.10.0
+     */
+    public static ?self $activeQuery = null;
+
     // Base config attributes
     // -------------------------------------------------------------------------
 
@@ -2846,7 +2853,12 @@ private function _applyCustomFieldParams(): void
                 if (isset($fieldsByHandle[$handle])) {
                     foreach ($fieldsByHandle[$handle] as $instances) {
                         $firstInstance = $instances[0];
-                        $condition = $firstInstance::queryCondition($instances, $fieldAttributes->$handle, $params);
+                        static::$activeQuery = $this;
+                        try {
+                            $condition = $firstInstance::queryCondition($instances, $fieldAttributes->$handle, $params);
+                        } finally {
+                            static::$activeQuery = null;
+                        }
 
                         // aborting?
                         if ($condition === false) {

src/fields/BaseRelationField.php

@@ -188,6 +188,7 @@ public static function queryCondition(array $instances, mixed $value, array &$pa
         }
 
         if (!empty($value)) {
+            $siteId = ElementQuery::$activeQuery?->siteId;
             $parser = new ElementRelationParamParser([
                 'fields' => [
                     $field->handle => $field,
@@ -196,7 +197,7 @@ public static function queryCondition(array $instances, mixed $value, array &$pa
             $condition = $parser->parse([
                 'targetElement' => $value,
                 'field' => $field->handle,
-            ]);
+            ], $siteId !== '*' ? $siteId : null);
             if ($condition !== false) {
                 $conditions[] = $condition;
             }