fix #409 - nullable large binary fields

ormar/models/mixins/save_mixin.py

@@ -1,3 +1,4 @@
+import base64
 import uuid
 from typing import (
     Any,
@@ -51,6 +52,7 @@ class SavePrepareMixin(RelationMixin, AliasMixin):
         new_kwargs = cls._remove_not_ormar_fields(new_kwargs)
         new_kwargs = cls.substitute_models_with_pks(new_kwargs)
         new_kwargs = cls.populate_default_values(new_kwargs)
+        new_kwargs = cls.reconvert_str_to_bytes(new_kwargs)
         new_kwargs = cls.translate_columns_to_aliases(new_kwargs)
         return new_kwargs
 
@@ -144,6 +146,36 @@ class SavePrepareMixin(RelationMixin, AliasMixin):
                     model_dict.pop(field, None)
         return model_dict
 
+    @classmethod
+    def reconvert_str_to_bytes(cls, model_dict: Dict) -> Dict:
+        """
+        Receives dictionary of model that is about to be saved and changes
+        all bytes fields that are represented as strings back into bytes.
+
+        :param model_dict: dictionary of model that is about to be saved
+        :type model_dict: Dict
+        :return: dictionary of model that is about to be saved
+        :rtype: Dict
+        """
+        bytes_fields = {
+            name
+            for name, field in cls.Meta.model_fields.items()
+            if field.__type__ == bytes
+        }
+        bytes_base64_fields = {
+            name
+            for name, field in cls.Meta.model_fields.items()
+            if field.represent_as_base64_str
+        }
+        for key, value in model_dict.items():
+            if key in bytes_fields and isinstance(value, str):
+                model_dict[key] = (
+                    value.encode("utf-8")
+                    if key not in bytes_base64_fields
+                    else base64.b64decode(value)
+                )
+        return model_dict
+
     @classmethod
     def populate_default_values(cls, new_kwargs: Dict) -> Dict:
         """