How to keep the 'this' context within a nested function

I need to access my object prototype function within the inner function oninit. The below code does work but what I need is to have the same context as self within the init function so I can call the prototype function like this.loadData.

Please assist.

define([
    'require',
    'lodash-compat',
    'BaseWidget'
],
function (require, _, BaseWidget) {
    var Widget = function (options) {
        this.set('url', '/some/endpoint/url');  
        var self = this;

        _.assign(options, {
            oninit: function () {
                self.loadData.call(this);
            },
            oncomplete: function () {
                //do something here
            }
        });
        self.ractive = new BaseWidget(options);
    };
    Widget.prototype = {
        loadData: function () {
            var url = this.get('url');
            // my async request
        }
    };
    return {
        initRactiveWidget: function (options) {
            return new Widget(options);
        }
    };
});