how to write the test class of the following apex class

Question

Here is my apex class, I would like to create the test class of it so that its coverage code must be 100%.Can anyone help me to create its test class.

@RestResource(urlMapping='/v2/getUpsell/*')

global with sharing class REST_Upsell_Controller {

    @HttpPost
    global static String processUpsell(String orderId){       

        ChargentOrders__ChargentOrder__c order = [SELECT Id, ChargentOrders__Charge_Amount__c, ChargentOrders__Tax__c, ChargentOrders__Subtotal__c FROM ChargentOrders__ChargentOrder__c WHERE Id = :orderId];
        order.ChargentOrders__Subtotal__c = 600.66;
        try {
            update  order;
            return 'Upsell Added'; 
        } catch (DmlException e) {

            return 'Upsell Not Added';
        }
    }   
}

Answer 1

What I have below is not 100% since it doesn't cover the update failing, and it also doesn't have any system.assert() methods, so it isn't even a proper test, but this should help get you started at least.

@isTest
public class TestRest{

    @testSetup
    public static void insertAnOrder(){
        Account a = new Account(name='AnAccount');
        insert a;
        insert new Order(name='AnOrder', AccountId=a.Id, EffectiveDate=system.today(), Status='Draft');
    }

    public static @isTest void testRest(){
        Id OrderId = [SELECT Id FROM Order LIMIT 1].Id;
        System.RestContext.request = new RestRequest();
        RestContext.request.requestURI = '/v2/getUpsell/';
        REST_Upsell_Controller.processUpsell(OrderId);
    }

}