Here is the code.

Code:
Ext.application({
    name : 'Fiddle',


    launch : function() {
        var session = new Ext.data.Session();
        Ext.define('EventType', {
            extend: 'Ext.data.Model',


            idProperty: 'eventTypeId',


            fields: [
                {name: 'eventTypeId'}
            ]
        });


        Ext.create('Ext.data.Store', {
            session: session,
            model: 'EventType',
            data : [
                {eventTypeId: 'A'},
                {eventTypeId: 'B'},
                {eventTypeId: 'C'}
            ]
        });


        Ext.define('Event', {
            extend: 'Ext.data.Model',


            idProperty: 'eventId',


            fields: [
                {name: 'eventId'},
                {
                    name: 'eventTypeId',
                    type: 'string',
                    reference: {
                        type: 'EventType',
                        role: 'EventType',
                        association: 'EventsByType',
                        inverse: 'events'
                    }
                },
            ]
        });


        var s= Ext.create('Ext.data.Store', {
            session: session,
            model: 'Event',
            data : [
                {eventId:1, eventTypeId: 'A'},
                {eventId:2, eventTypeId: 'A'},
                {eventId:3,eventTypeId: 'C'}
            ]
        });


        var m = s.getAt(0);


        console.log(m.getAssociatedData(), 'EMPTY');
        console.log(m.getEventType(), 'EXISTS');
    }
});

https://fiddle.sencha.com/#view/editor&fiddle/31gm


Models has links but no associated data.