Collection#fetch updates existing model instances (vs replacing)

[benvinegar]

If I create a new model, and then call fetch, it will update that model's attributes.

If I create a collection of models, and then call fetch, it won't update the existing models – it will replace them with new instances.

This is painful because I cannot cache references to the collection's model instances, as the references become invalid once a fetch occurs. Instead I'd have to cache references to the collection, and do Collection#get every time I want to access the model. Maybe that's acceptable, but it seems reasonable that a collection should be able to perform a fetch/update without invalidating existing model instances.

[tbranyen]

I could see an option being created for this, but the default behavior should still remain reset, imo.

[iros]

How would you want to handle "add" events to the collection if you're only updating a model? Would you want your change event to fire instead? I can see situations where both would be valid behaviors (throw-away views vs carefully managed views etc.)
I think having a flag for this would be pretty swell.

[braddunbar]

Yes, I've implemented something like this, adding/removing as necessary instead of replacing. I call it a "soft" reset. It doesn't support updating the models but it certainly could. Very handy.

[tomasztunik]

We've used custom extension to do exactly that in Wunderkit as well - 👍 from us on this one - and yes - it should be as an option rather then default behavior

edit: hm I'm wondering if we could implement kind of "sync" behavior here - for example a fetch returns same collection - but some models are gone, some are changed and some are added. Now it should not only check vs models if they already exist but it should also check if any models weren't removed so they should get removed from the collection as well. Something to think about :)

[vincentbriglia]

There used to be a pull request, I have not checked whether the snippet works with Backbone 0.9.1 but it should get you on track dalyons@0af0e23

[ruudk]

@dalyons Why is there no PR for this awesome feature? Because it works like a charm!

[jashkenas]

I'd be glad to entertain a pull request for "smart" resets, but -- they're already easy to accomplish with the tools backbone gives you: set(), changedAttributes(), _.isEqual, etc.

Furthermore, you'd have to deal with a number of API choices, most of which have no "right" answer:

• If a model is present in the collection, but not in the response, do you delete it?
• If a model is present in the response, but not in the collection, do you add it?
• How does it work for infinite-scroll-style collections, where you're only fetching a window of models from the server?