--- a/cubicweb/server/migractions.py	Wed May 25 09:51:51 2016 +0200
+++ b/cubicweb/server/migractions.py	Tue Jun 07 10:05:30 2016 +0200
@@ -1369,7 +1369,7 @@
             getattr(entity, attribute)
             storage.migrate_entity(entity, attribute)
             # remove from entity cache to avoid memory exhaustion
-            del entity.cw_attr_cache[attribute]
+            entity.cw_attr_cache.pop(attribute, None)
             pb.update()
         print()
         source.set_storage(etype, attribute, storage)

--- a/cubicweb/server/sources/storages.py	Wed May 25 09:51:51 2016 +0200
+++ b/cubicweb/server/sources/storages.py	Tue Jun 07 10:05:30 2016 +0200
@@ -157,12 +157,13 @@
             entity._cw_dont_cache_attribute(attr, repo_side=True)
         else:
             binary = entity.cw_edited.pop(attr)
-            fd, fpath = self.new_fs_path(entity, attr)
-            # bytes storage used to store file's path
-            binary_obj = Binary(fpath if PY2 else fpath.encode('utf-8'))
-            entity.cw_edited.edited_attribute(attr, binary_obj)
-            self._writecontent(fd, binary)
-            AddFileOp.get_instance(entity._cw).add_data(fpath)
+            if binary is not None:
+                fd, fpath = self.new_fs_path(entity, attr)
+                # bytes storage used to store file's path
+                binary_obj = Binary(fpath if PY2 else fpath.encode('utf-8'))
+                entity.cw_edited.edited_attribute(attr, binary_obj)
+                self._writecontent(fd, binary)
+                AddFileOp.get_instance(entity._cw).add_data(fpath)
         return binary
 
     def entity_updated(self, entity, attr):
@@ -259,13 +260,14 @@
     def migrate_entity(self, entity, attribute):
         """migrate an entity attribute to the storage"""
         entity.cw_edited = EditedEntity(entity, **entity.cw_attr_cache)
-        self.entity_added(entity, attribute)
-        cnx = entity._cw
-        source = cnx.repo.system_source
-        attrs = source.preprocess_entity(entity)
-        sql = source.sqlgen.update('cw_' + entity.cw_etype, attrs,
-                                   ['cw_eid'])
-        source.doexec(cnx, sql, attrs)
+        binary = self.entity_added(entity, attribute)
+        if binary is not None:
+            cnx = entity._cw
+            source = cnx.repo.system_source
+            attrs = source.preprocess_entity(entity)
+            sql = source.sqlgen.update('cw_' + entity.cw_etype, attrs,
+                                       ['cw_eid'])
+            source.doexec(cnx, sql, attrs)
         entity.cw_edited = None
 
 

--- a/cubicweb/server/test/data-migractions/schema.py	Wed May 25 09:51:51 2016 +0200
+++ b/cubicweb/server/test/data-migractions/schema.py	Tue Jun 07 10:05:30 2016 +0200
@@ -150,6 +150,7 @@
         })
     description = String()
     firstname = String(fulltextindexed=True, maxsize=64)
+    photo = Bytes()
 
     concerne = SubjectRelation('Affaire')
     connait = SubjectRelation('Personne')

--- a/cubicweb/server/test/unittest_migractions.py	Wed May 25 09:51:51 2016 +0200
+++ b/cubicweb/server/test/unittest_migractions.py	Tue Jun 07 10:05:30 2016 +0200
@@ -20,17 +20,20 @@
 from datetime import date
 import os, os.path as osp
 from contextlib import contextmanager
+import tempfile
 
-from logilab.common.testlib import unittest_main, Tags, tag
+from logilab.common.testlib import unittest_main, Tags, tag, with_tempdir
 from logilab.common import tempattr
 
 from yams.constraints import UniqueConstraint
 
-from cubicweb import ConfigurationError, ValidationError, ExecutionError
+from cubicweb import (ConfigurationError, ValidationError,
+                      ExecutionError, Binary)
 from cubicweb.devtools import startpgcluster, stoppgcluster
 from cubicweb.devtools.testlib import CubicWebTC
 from cubicweb.server.sqlutils import SQL_PREFIX
 from cubicweb.server.migractions import ServerMigrationHelper
+from cubicweb.server.sources import storages
 
 import cubicweb.devtools
 
@@ -488,7 +491,7 @@
                 'X ordernum O')]
             expected = [u'nom', u'prenom', u'sexe', u'promo', u'ass', u'adel', u'titre',
                         u'web', u'tel', u'fax', u'datenaiss', u'test', u'tzdatenaiss',
-                        u'description', u'firstname',
+                        u'description', u'firstname', u'photo',
                         u'creation_date', u'cwuri', u'modification_date']
             self.assertEqual(expected, rinorder)
 
@@ -755,6 +758,29 @@
             mh.drop_relation_definition('Note', 'ecrit_par', 'CWUser')
             self.assertFalse(mh.sqlexec('SELECT * FROM cw_Note WHERE cw_ecrit_par IS NOT NULL'))
 
+    @with_tempdir
+    def test_storage_changed(self):
+        with self.mh() as (cnx, mh):
+            john = mh.cmd_create_entity('Personne', nom=u'john',
+                                        photo=Binary(b'something'))
+            bill = mh.cmd_create_entity('Personne', nom=u'bill')
+            mh.commit()
+            bfs_storage = storages.BytesFileSystemStorage(tempfile.tempdir)
+            storages.set_attribute_storage(self.repo, 'Personne', 'photo', bfs_storage)
+            mh.cmd_storage_changed('Personne', 'photo')
+            bob = mh.cmd_create_entity('Personne', nom=u'bob')
+            bffss_dir_content = os.listdir(tempfile.tempdir)
+            self.assertEqual(len(bffss_dir_content), 1)
+            john.cw_clear_all_caches()
+            self.assertEqual(john.photo.getvalue(),
+                             osp.join(tempfile.tempdir, bffss_dir_content[0]))
+            bob.cw_clear_all_caches()
+            self.assertIsNone(bob.photo)
+            bill.cw_clear_all_caches()
+            self.assertIsNone(bill.photo)
+            storages.unset_attribute_storage(self.repo, 'Personne', 'photo')
+
+
 class MigrationCommandsComputedTC(MigrationTC):
     """ Unit tests for computed relations and attributes
     """