Fixed `yii\db\ActiveRelationTrait::populateRelation()` fails when `link` refers to string convertable object attribute, like `\MongoId`

framework/CHANGELOG.md

@@ -31,6 +31,7 @@ Yii Framework 2 Change Log
 - Bug #8082: Fixed `yii\db\BaseActiveRecord::getAttributeLabel()` return wrong label for related attribute, if several relations in chain share same name (klimov-paul)
 - Bug #8149: Fixed `yii\db\BaseActiveRecord::updateCounters()` fails for new record saved with counter attribute not set (klimov-paul)
 - Bug #8158: Avoid exception when detecting console screen size fails on windows (EliasZ)
+- Bug #8165: Fixed `yii\db\ActiveRelationTrait::populateRelation()` fails when `link` refers to string convertable object attribute, like `\MongoId` (klimov-paul)
 - Enh #1468: Added ability to specify hints for model attributes via `attributeHints()` method (klimov-paul)
 - Enh #3376: Added `yii\validators\EachValidator`, which allows validation of the array attributes (klimov-paul)
 - Enh #6442: Improved error message on `FileHelper::createDirectory()` to include the path name of the directory (cebe)

framework/db/ActiveRelationTrait.php

@@ -483,19 +483,20 @@ trait ActiveRelationTrait
      */
     private function getModelKey($model, $attributes)
     {
-        if (count($attributes) > 1) {
-            $key = [];
-            foreach ($attributes as $attribute) {
-                $key[] = $model[$attribute];
+        $key = [];
+        foreach ($attributes as $attribute) {
+            $value = $model[$attribute];
+            if (is_object($value) && method_exists($value, '__toString')) {
+                // ensure matching to special objects, which are convertable to string, for cross-DBMS relations, for example: `|MongoId`
+                $value = $value->__toString();
             }
-
+            $key[] = $value;
+        }
+        if (count($key) > 1) {
             return serialize($key);
-        } else {
-            $attribute = reset($attributes);
-            $key = $model[$attribute];
-
-            return is_scalar($key) ? $key : serialize($key);
         }
+        $key = reset($key);
+        return is_scalar($key) ? $key : serialize($key);
     }
 
     /**