How do I get ExtJS 5 AjaxProxy to save?

I'm upgrading an app to ExtJS 5, and I can't seem to get a grid using RowEditing to POST the edited record back to my server.

Ext.define("MyRecordDef", { extend: "Ext.data.Model" });

var MyEditor = Ext.create('Ext.grid.plugin.RowEditing', { clicksToEdit: 1 });

var MyStore = new Ext.data.Store({
        model:      "MyRecordDef",
        autoSync:   true,
        proxy: {
            type:           "ajax",
            url:            "ajaxurl.aspx",
            batchActions:   false,
            extraParams:    { Command: 'Save' },
            reader:         { type: "json", rootProperty: "rows" },
            writer:         { type: "json", encode: true, writeAllFields: true }
        }
    });

var MyGrid = new Ext.grid.GridPanel({
    store:      MyStore,
    plugins:    [ MyEditor ],
    columns:        [
        {
        id:             "fieldtoedit",
        dataIndex:      "fieldtoedit",
        editor:         new Ext.form.NumberField()
        }
    ]
});

The row editor comes up, I'm able to change the value and click the Update button, but then nothing happens. No request is made, no errors logged in the console.

I added the following:

MyGrid.on('edit', function (e) {
    alert('You are Editing ' + e.context.record.id);
    MyStore.sync();  // attempt to force a sync
});

I get the alert with the correct record number, but still no AJAX request. It's like ExtJS is completely ignoring the writer.

I don't have different endpoints for each CRUD operation, so I'm not using the api config option, it should be using the url.

First of all you must define rootProperty on writer when you use encode: true. Then after adding fields to MyRecordDef requests are sended.

Working sample: http://jsfiddle.net/jjVwR/3/ (saving don't work, but you can see on console that request is send)