How to shallow copy app engine model instance to create new instance?
I want to implement a simple VersionedModel base model class for my app engine app. I'm looking for a pattern that does not involve explicitly choosing fields to copy.
I am trying out something like this, but it is to hacky for my taste and did not test it in the production environment yet.
class VersionedModel(BaseModel):
is_history_copy = db.BooleanProperty(default=False)
version = db.IntegerProperty()
created = db.DateTimeProperty(auto_now_add=True)
edited = db.DateTimeProperty()
user = db.UserProperty(auto_current_user=True)
def put(self, **kwargs):
if self.is_history_copy:
if self.is_saved():
raise Exception, "History copies of %s are not allowed to change" % type(self).__name__
return super(VersionedModel, self).put(**kwargs)
if self.version is None:
self.version = 1
else:
self.version = self.version +1
se开发者_StackOverflowlf.edited = datetime.now() # auto_now would also affect copies making them out of sync
history_copy = copy.copy(self)
history_copy.is_history_copy = True
history_copy._key = None
history_copy._key_name = None
history_copy._entity = None
history_copy._parent = self
def tx():
result = super(VersionedModel, self).put(**kwargs)
history_copy._parent_key = self.key()
history_copy.put()
return result
return db.run_in_transaction(tx)
Does anyone have a simpler cleaner solution for keeping history of versions for app engine models?
EDIT: Moved copy
out of tx. Thx @Adam Crossland for the suggestion.
Take a look at the properties static method on Model classes. With this, you can get a list of properties, and use that to get their values, something like this:
@classmethod
def clone(cls, other, **kwargs):
"""Clones another entity."""
klass = other.__class__
properties = other.properties().items()
kwargs.update((k, p.__get__(other, klass)) for k, p in properties)
return cls(**kwargs)
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