diff --git a/src/Relation/HasOne.php b/src/Relation/HasOne.php
index ff2d2a0..6332e39 100644
--- a/src/Relation/HasOne.php
+++ b/src/Relation/HasOne.php
@@ -83,7 +83,7 @@ public function render(Registry $registry): void
         $table = $registry->getTableSchema($target);
 
         if ($this->options->get(self::INDEX_CREATE)) {
-            $table->index([$outerField->getColumn()]);
+            $table->index([$outerField->getColumn()])->unique();
         }
 
         if ($this->options->get(self::FK_CREATE)) {