From e20799d79a6899ed617040123472e1f69b9827a3 Mon Sep 17 00:00:00 2001
From: brandonkelly <brandon@pixelandtonic.com>
Date: Tue, 15 Oct 2024 14:06:22 -0700
Subject: [PATCH] Fixed #15890

---
 CHANGELOG.md                     |  1 +
 src/fields/BaseRelationField.php | 59 +++++++++++++++++++++-----------
 2 files changed, 40 insertions(+), 20 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index c8060be1de6..bc204fbff84 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@
 - Fixed a bug where Craft wasn’t auto-detecting interactive terminals on Windows.
 - Fixed a bug where element actions were allowed on nested entries when viewing a revision. ([#15879](https://github.com/craftcms/cms/pull/15879))
 - Fixed a bug where element error summaries weren’t linking to recursively-nested Matrix fields properly. ([#15797](https://github.com/craftcms/cms/issues/15797))
+- Fixed a bug where eager-loaded relation fields were loading all related elements across all instances of the field. ([#15890](https://github.com/craftcms/cms/issues/15890))
 - Fixed a privilege escalation vulnerability.
 
 ## 5.4.7.1 - 2024-10-09
diff --git a/src/fields/BaseRelationField.php b/src/fields/BaseRelationField.php
index db062499c17..1fb2ca13f5c 100644
--- a/src/fields/BaseRelationField.php
+++ b/src/fields/BaseRelationField.php
@@ -958,27 +958,46 @@ public function getEagerLoadingMap(array $sourceElements): array|null|false
     {
         $sourceSiteId = $sourceElements[0]->siteId;
 
-        // Get the source element IDs
-        $sourceElementIds = array_map(fn(ElementInterface $element) => $element->id, $sourceElements);
+        $map = [];
+        $missingSourceElementIds = [];
 
-        // Return any relation data on these elements, defined with this field
-        $map = (new Query())
-            ->select(['sourceId as source', 'targetId as target'])
-            ->from([DbTable::RELATIONS])
-            ->where([
-                'and',
-                [
-                    'fieldId' => $this->id,
-                    'sourceId' => $sourceElementIds,
-                ],
-                [
-                    'or',
-                    ['sourceSiteId' => $sourceSiteId],
-                    ['sourceSiteId' => null],
-                ],
-            ])
-            ->orderBy(['sortOrder' => SORT_ASC])
-            ->all();
+        foreach ($sourceElements as $sourceElement) {
+            $rawValue = $sourceElement->getBehavior('customFields')->{$this->handle} ?? null;
+            if ($rawValue instanceof ElementQueryInterface) {
+                $rawValue = $rawValue->where['elements.id'] ?? null;
+            }
+            if (is_array($rawValue)) {
+                foreach ($rawValue as $targetElementId) {
+                    $map[] = ['source' => $sourceElement->id, 'target' => $targetElementId];
+                }
+            } elseif ($this->isFirstInstance($sourceElement)) {
+                // The relation IDs aren't hardcoded yet and this is the first
+                // instance of this field in the field layout, so fetch the relations
+                // via the DB table
+                $missingSourceElementIds[] = $sourceElement->id;
+            }
+        }
+
+        // Are there any source elements that don't have hardcoded relation IDs yet?
+        if (!empty($missingSourceElementIds)) {
+            $missingMappingsQuery = (new Query())
+                ->select(['sourceId as source', 'targetId as target'])
+                ->from([DbTable::RELATIONS])
+                ->where([
+                    'and',
+                    [
+                        'fieldId' => $this->id,
+                        'sourceId' => $missingSourceElementIds,
+                    ],
+                    [
+                        'or',
+                        ['sourceSiteId' => $sourceSiteId],
+                        ['sourceSiteId' => null],
+                    ],
+                ])
+                ->orderBy(['sortOrder' => SORT_ASC]);
+            array_push($map, ...$missingMappingsQuery->all());
+        }
 
         $criteria = [];