Couple of changes

byostorage/apps.py

@@ -4,3 +4,4 @@ from django.apps import AppConfig
 class BYOStorageConfig(AppConfig):
     name = 'byostorage'
     verbose_name = 'Bring your own storage'
+    default_auto_field = 'django.db.models.AutoField'

byostorage/local_settings.py

@@ -5,11 +5,17 @@ BASE_DIR = Path(__file__).resolve().parent.parent
 
 MEDIA_ROOT = "media"
 
+DEBUG = True
+DEFAULT_AUTO_FIELD = 'django.db.models.AutoField'
+
+
 INSTALLED_APPS = [
     'django.contrib.auth',
     'django.contrib.contenttypes',
-    'byostorage'
+    'byostorage',
+    'example'
 ]
+
 SECRET_KEY = 'shh!'
 
 DATABASES = {

byostorage/models.py

@@ -1,9 +1,15 @@
 from django.conf import settings
 from django.db import models
-from django.core.files.storage import get_storage_class, FileSystemStorage
+from django.db.models.fields.files import FieldFile
+from django.core.exceptions import ObjectDoesNotExist
+from django.core.files.storage import Storage, get_storage_class
 from django.core.exceptions import ValidationError
 import json
 
+import logging
+logger = logging.getLogger(__name__)
+logger.setLevel(logging.DEBUG)
+
 STORAGE_CLASSES = getattr(settings, 'STORAGE_CLASSES', [])
 
 STORAGE_CLASSES.extend([
@@ -11,7 +17,7 @@ STORAGE_CLASSES.extend([
 ])
 
 try:
-    import storages
+    import storages.backends
     STORAGE_CLASSES.append('storages.backends.s3boto3.S3Boto3Storage')
 except ImportError:
     pass
@@ -24,6 +30,15 @@ def validate_json(value):
     except json.JSONDecodeError as e:
         raise ValidationError(e.msg)
 
+def resolve_instance(obj, relation):
+    for p in relation.split('__'):
+        obj = getattr(obj, p)
+
+        if callable(obj):
+            obj = obj()
+
+    return obj
+
 class UserStorage(models.Model):
     """ A user defined storage
     """
@@ -75,4 +90,45 @@ class UserStorage(models.Model):
         super(UserStorage, self).save(*args, **kwargs)
 
     def __str__(self):
-        return self.name
+        return self.name
+
+class BYOFieldFile(FieldFile):
+    
+    def __init__(self, instance, field, name):
+        logger.debug("BYOFieldFile(%r, %r, %r)", instance, field, name)
+        super().__init__(instance, field, name)
+        try:
+            self.storage = field.get_storage(instance)
+            logger.debug("Using BYO storage: %r", self.storage)
+        except ObjectDoesNotExist:
+            logger.debug("Unable to select BYO storage")
+            self.storage = None # trigger error if try to save etc
+
+class BYOStorageField(models.FileField):
+    attr_class = BYOFieldFile
+
+    def __init__(self, storage_instance, *args, **kwargs):
+        self.storage_instance = storage_instance
+        super().__init__(*args, **kwargs)
+
+    def deconstruct(self):
+        name, path, args, kwargs = super().deconstruct()
+        args = [self.storage_instance] + args
+        return name, path, args, kwargs
+
+    def get_storage(self, instance):
+
+        if callable(self.storage_instance):
+            storage = self.storage_instance(instance)
+        else:
+            storage = resolve_instance(instance, self.storage_instance)
+        
+        if not isinstance(storage, Storage):
+            raise RuntimeError("Not a storage instance")
+
+        return storage
+
+    #def formfield(self, **kwargs):
+    #    return BYOStorageFormField(**kwargs)
+
+