Backbonejs Model.url required? - rest

Question

I have an example setup to show my issue: http://jsfiddle.net/5xo4yzw2/1/
var root = "http://jsonplaceholder.typicode.com"
var Post = Backbone.Model.extend({
url: function(){
return this.get('url') || root + "/posts/" + this.get('id')
}
});
var Posts = Backbone.Collection.extend({
model: Post,
url: root + "/posts"
});
var posts = new Posts();
posts.fetch();
console.log("Call to collection fetch works")
var post = new Post({id: 1});
post.fetch();
console.log("Call to collection fetch works")
This example works, that is, both of the console.logs are executed without an exception being raised by .fetch(). It was my understanding, however, from both http://backbonejs.org/#Model-url and the book I am reading, that once I specify "/posts" as the collection url, the model url should be automatically resolved to [collection.url]/[id], though that is not the case. If I do not provide the url function in the model, an exception is raises upon fetching, stating the th url parameter must be provided.
Have i misunderstood something?

Answer 1

You haven't added your post model to your collection, in order for a model to use the collections URL it must be part of that collection.
For example
var post = new Post({id: 1});
posts.add(post);
post.fetch();
console.log("Call to collection fetch works")
Updated Fiddle
As #Artem Baranovskii pointed out, If you want to use a model outside a collection you should be using the urlRoot property. From the documentation
Specify a urlRoot if you're using a model outside of a collection, to
enable the default url function to generate URLs based on the model
id. "[urlRoot]/id" Normally, you won't need to define this. Note that
urlRoot may also be a function.

Answer 2

If you're using a model outside of the collection you could use urlRoot like the following:
var root = "http://jsonplaceholder.typicode.com"
var Posts = Backbone.Model.extend({
urlRoot: function () {
return this.get('url') || (root + "/posts/" + this.get('id'))
}
});
var posts = new Posts();
posts.fetch();
console.log("Call to collection fetch works")
When you execute fetch the model tries to get url by the several approaches here