UniqueEntityValidator: Retrieve at max two entities for unique entity check.

Fixes: #43254

Author: AndreasA

CHANGELOG.md

@@ -6,6 +6,7 @@ CHANGELOG
 
  * Add `DoctrineOpenTransactionLoggerMiddleware` to log when a transaction has been left open
  * Deprecate `PdoCacheAdapterDoctrineSchemaSubscriber` and add `DoctrineDbalCacheAdapterSchemaSubscriber` instead
+ * `UniqueEntity` constraint retrieves a maximum of two entities if the default repository method is used.
 
 5.3
 ---

Validator/Constraints/UniqueEntityValidator.php

@@ -134,7 +134,18 @@ public function validate($entity, Constraint $constraint)
             $repository = $em->getRepository(\get_class($entity));
         }
 
-        $result = $repository->{$constraint->repositoryMethod}($criteria);
+        $arguments = [$criteria];
+
+        /* If the default repository method is used, it is always enough to retrieve at most two entities because:
+         * - No entity returned, the current entity is definitely unique.
+         * - More than one entity returned, the current entity cannot be unique.
+         * - One entity returned the uniqueness depends on the current entity.
+         */
+        if ('findBy' === $constraint->repositoryMethod) {
+            $arguments = [$criteria, null, 2];
+        }
+
+        $result = $repository->{$constraint->repositoryMethod}(...$arguments);
 
         if ($result instanceof \IteratorAggregate) {
             $result = $result->getIterator();