ONETOMANY should not be InputRequired too.

flask_admin/contrib/sqla/form.py

@@ -106,10 +106,8 @@ class AdminModelConverter(ModelConverterBase):
         kwargs['label'] = self._get_label(prop.key, kwargs)
         kwargs['description'] = self._get_description(prop.key, kwargs)
 
-        if column.nullable:
+        if column.nullable or prop.direction.name != 'MANYTOONE':
             kwargs['validators'].append(validators.Optional())
-        elif prop.direction.name != 'MANYTOMANY':
-            kwargs['validators'].append(validators.InputRequired())
 
         # Contribute model-related parameters
         if 'allow_blank' not in kwargs: