Move ObjectIdMixin to objectid.py.

Author: WaVEV

django_mongodb/fields/auto.py

@@ -3,7 +3,7 @@
 from django.db.models.fields import AutoField
 from django.utils.functional import cached_property
 
-from .objectid_mixin import ObjectIdMixin
+from .objectid import ObjectIdMixin
 
 
 class ObjectIdAutoField(ObjectIdMixin, AutoField):

django_mongodb/fields/objectid.py

@@ -2,15 +2,29 @@
 from bson.errors import InvalidId
 from django.core import exceptions
 from django.db.models.fields import Field
+from django.utils.translation import gettext_lazy as _
 
-from .objectid_mixin import ObjectIdMixin
+
+class ObjectIdMixin:
+    default_error_messages = {
+        "invalid": _("“%(value)s” value must be an Object Id."),
+    }
+    description = _("Object Id")
+
+    def db_type(self, connection):
+        return "objectId"
+
+    def rel_db_type(self, connection):
+        return "objectId"
 
 
 class ObjectIdField(ObjectIdMixin, Field):
     def get_internal_type(self):
         return "ObjectIdField"
 
     def to_python(self, value):
+        if value is None:
+            return value
         try:
             return ObjectId(value)
         except (TypeError, InvalidId):