Add get_prep_value.

Author: WaVEV

django_mongodb/fields/objectid.py

@@ -1,4 +1,4 @@
-from bson import ObjectId
+from bson import ObjectId, errors
 from bson.errors import InvalidId
 from django.core import exceptions
 from django.db.models.fields import Field
@@ -34,6 +34,14 @@ def to_python(self, value):
                 params={"value": value},
             ) from None
 
+    def get_prep_value(self, value):
+        if value is None:
+            return None
+        try:
+            return ObjectId(value)
+        except (errors.InvalidId, TypeError) as e:
+            raise ValueError(f"Field '{self.name}' expected an ObjectId but got {value!r}.") from e
+
     def deconstruct(self):
         name, path, args, kwargs = super().deconstruct()
         if path.startswith("django_mongodb.fields.objectid"):