Check out example codes for "if entity.is Transient() Update Mvc 5 c#". It will help you in understanding the concepts better.

Code Example 1

[HttpGet]
    public virtual ActionResult Edit(int id)
    {
        Employee entity = GetEntityById(id);
        EmployeeEditViewModel model = new EmployeeEditViewModel();

        Mapper.Map(source, destination);            

        return View("Edit", model);
    }

    [HttpPost]
    public virtual ActionResult Edit(EmployeeEditViewModel model)
    { 
        if (ModelState.IsValid)
        {
            Employee entity = GetEntityById(model.Id);

            entity = Mapper.Map(model, entity);               
            EntitiesRepository.Save(entity);

            return GetIndexViewActionFromEdit(model);
        }           

        return View("Edit", model);
    }

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