SOURCE

cola(function(model){
 model.dataType({
            name: "Company",
            properties: {
                depts: {
                    provider: {
                        url: "http://open-data.cola-ui.com/service/company/{{@id}}/depts"
                    },
                  	dataType:{
                      properties: {
                        employees:{
                          provider: {
                        url: "http://open-data.cola-ui.com/service/dept/{{@id}}/employees"
                    },
                        }
                      }
                    }
                }
            }
        });
      

        model.describe("companies", {
            dataType: "Company",
            provider: "http://open-data.cola-ui.com/service/company"
        });

        model.widgetConfig({
            employeeTree: {
                bind: {
                    expression: "company in companies",
                   textProperty:"name",
                    child: {
                        textProperty:"name",
                        expression: "dept in company.depts",
                      child: {
                        textProperty:"name",
                        expression: "employee in dept.employees"
                    }
                    }
                },
                height: "100%"
            }
        });
})

<c-tree  id="employeeTree"></c-tree>
<br>