App Engine: how would you... snapshotting entities
- by Andrew B.
Let's say you have two kinds, Message and Contact, related by a
db.ListProperty of keys on Message. A user creates a message, adds
some contacts as recipients, and emails the message. Later, the user
deletes one of the contact entities that was a recipient of the
message. Our application should delete the appropriate Contact
entity, but we want to preserve the original recipient list for the
message that was sent for the user's records. In essence, we want a
snapshot of the message entity at the time it was sent. If we naively
delete the contact entity, though, we lose snapshot integrity; if not,
we are left with an invalid key.
How would you handle this situation,
either in controller logic or model changes?
class User(db.Model):
email = db.EmailProperty(required=True)
class Contact(db.Model):
email = db.EmailProperty(required=True)
user = db.ReferenceProperty(User, collection_name='contacts')
class Message(db.Model):
recipients = db.ListProperty(db.Key) # contacts
sender = db.ReferenceProperty(User, collection_name='messages')
body = db.TextProperty()
is_emailed = db.BooleanProperty(default=False)
