google app engine - Cannot assign key to blobstore.BlobReferenceProperty() -

i trying upload photos blobstore. code similar example here

class image(ndb.model):     date = ndb.datetimeproperty(auto_now_add=true)     user = ndb.userproperty()     blob_key = blobstore.blobreferenceproperty() 

in uploadhandler following:

blob_info = self.get_uploads('file')[0] image = image() image.user = users.get_current_user() image.blob_key = blob_info.key() 

however, last line results in following error:

typeerror: cannot concatenate 'str' , 'nonetype' objects 

this driving me crazy. can upload picture, can't store key! know might wrong?

you have t revert data model db if want use blobreferenceproperty, not documented, if want use ndb need use blobkeyproperty mentioned voscausa. find db model simpler use ndb , don't hype round ndb

class image(db.model):     user = db.userproperty()     blob_key = blobstore.blobreferenceproperty()