Question

I call save using this:

console.log(this.model.isNew());
console.log(this.model);

this.model.save({}, {
    success: function (model, response, options) {
        console.log(response);
    },
    error: function (model, xhr, options) {
        console.log(xhr.result.Errors);
    }   
});

The isNew() returns false. But the output of this.model has an ID of 0. (this.model.id is 0 as well)

My url is url: ROOTAREA + "/Expenses/Entry/",

Updating works fine, and uses PUT as expected.

Edit : here’s part of my model:

   defaults: function () {
        return {
            DocumentDate: "",
            JobNo_: "",
            PhaseCode: "",
            WorkTypeCode: "",
            Description: "",
            Quantity: 0,
            UnitCost: 0,
            ExpenseCurrencyCode: "",
            ReimbursementCurrencyCode: "",
            UnitofMeasureCode: "DIEM",
            LineNo_: 0
        };
    },
    idAttribute: "LineNo_",

Answer 1

ID should not even exist for a new entry.
The issue is in the part you didn’t show – in the part where you instantiate, create and populate the model.

Here is a quote from the Backbone documentation:

If the model does not yet have an id, it is considered to be new.

It is clear from your code that you are assigning an id attribute.
Your backend should be doing that.
And since you are doing it on a client, backbone presumes it it not new, and uses PUT