add auto_now field to DateTimeField about schematics

[robspychala]

app engine does a good job with this.

(from http://code.google.com/appengine/docs/python/datastore/typesandpropertyclasses.html#DateTimeProperty)

If auto_now is True, the property value is set to the current time whenever the model instance is stored in the datastore, overwriting the property's previous value. This is useful for tracking a "last modified" date and time for a model instance.

If auto_now_add is True, the property value is set to the current time the first time the model instance is stored in the datastore, unless the property has already been assigned a value. This is useful for storing a "created" date and time for a model instance.

[gone]

Yeah this is really helpful in django apps. I'd like to see it here too

[alexanderdean]

I think it's a great feature, but isn't auto_now and auto_now_add a little cryptic? From an ORM perspective (see e.g. PHP Doctrine, Ruby) these would be more likely to be called create_timestamp and update_timestamp or similar...

[alexanderdean]

Sorry I spoke too soon - DictShield doesn't have any way of knowing if a record is new or existing, so auto_now_add becomes a non-issue. I think auto_now is a fine name

[gone]

I took a look at this last night. Dictshield doesn't have a pre-persist hook. Which it shouldn't, since it doesn't even really include the notion of persistence. How should this work? When you create a dictshield with auto_now=True the field is set? Then when you import data from a persistence layer, it would blow it away? That's what makes sense to me. Thoughts?

[alexanderdean]

Agree with all of that @gone - especially that DictShield should not have a pre-persist hook. Keen that DictShield stays usage-agnostic (I don't use it for persistence at all - just for typesafe JSON (un)marshalling)...

[j2labs]

I like the idea of auto_now, but you can also set the DateTimeField's default value to datetime.datetime.now, without the parenthesis, and it will call that function every time a model is instantiated. It's perhaps nto quite as explicit but you can still set the field to default to the time of instantiation.

I'm going to close this for now unless a good argument for being explicit is made. :)