Merge local Backbone collection with server - backbone.js

Question

I have a local Backbone collection:
var collection = new Backbone.Collection([ { greeting: "hi" }, { greeting: "bye" } ]);
I understand that when I run collection.fetch, Backbone will run collection.set on the results. I need to merge in the response from the server, however. Say the response is:
[ { id: "2", greeting: "hi", name: "Bob" } ]
I would like the resulting collection, after the merge, to be:
[ { id: "2", greeting: "hi", name: "Bob" }, { greeting: "bye" } ]
I understand Backbone already attempts to do some merging here, but if I set the example response above, no merge happens and a new model gets added instead. I assume this is because it merges by id, and here we do not have any ids (in the local collection). In this case, greeting is my unique identifier / key.
The reason I am trying to do this is because I have a local collection and I simply want to see what already exists from that collection (using the key greeting) and merge any findings in.

Answer 1

My solution:
feeds.fetch({
add: false,
remove: false,
merge: false,
data: params,
success: function (feeds, response) {
// Merge any matches
_.each(response.results, function (result) {
_.each(feeds.models, function (feed) {
// We have to `parse` the result before setting it, as Model#set does
// not automatically run `parse` (Collection#set does).
result = feed.parse(result)
if (feed.get('rssUrl') === result.rssUrl) feed.set(result)
})
})
cb(feeds)
}
})

Answer 2

You can tell backbone to use a different key for the id attribute on your model:
GreetingModel = Backbone.Model.extend({
idAttribute: "greeting"
});
GreetingCollection = Backbone.Collection.extend({
model: GreetingModel
});
http://backbonejs.org/#Model-idAttribute
Edit: I suppose you could use two separate collections for local and server side.
var localCollection = new Backbone.Collection([ { greeting: "hi" }, { greeting: "bye" } ]);
ServerCollection = Backbone.Collection.extend({
url: "/api/"
...
});
var serverCollection = new ServerCollection({});
serverCollection.on("reset", function() {
localCollection.each(function(localModel) {
var greeting = localModel.get("greeting");
serverModel = serverCollection.findWhere({greeting: greeting});
if(serverModel) {
localModel.set(serverModel.attributes);
}
});
});
serverCollection.fetch();