feat: unset default BelongsToMany relations when not loaded

Author: laylatichy

packages/database/src/Mappers/SelectModelMapper.php

@@ -48,13 +48,13 @@ public function map(mixed $from, mixed $to): array
 
         foreach ($objects as $i => $object) {
             foreach ($model->getRelations() as $relation) {
-                // When a nullable BelongsTo relation wasn't loaded, we need to make sure to unset it if it has a default value.
+                // When a relation with a default value wasn't loaded, we need to unset it.
                 // If we wouldn't do this, the default value would overwrite the "unloaded" value on the next time saving the model
-                if (! $relation instanceof BelongsTo) {
+                if ($relation instanceof BelongsTo && ! $relation->property->isNullable()) {
                     continue;
                 }
 
-                if (! $relation->property->isNullable()) {
+                if (! $relation instanceof BelongsTo && ! $relation instanceof BelongsToMany) {
                     continue;
                 }