How to make simple calculations using model items and an input from a form in ASP.net MVC 3?

Question

Am new to programming and ASP.net MVC 3 so don't be surprised by my lack of knowledge on this.. Okay, I want to multiply two decimals, One decimal comes from the form that a user fills and the other decimal comes from the Model class (gets it from the database).

I have two Model classes called RATE & PROJECTMATERIAL . The RATE class has an item called Amount that states the amount of a Rate and the PROJECTMATERIAL class has an item quantity. The classes are related and i want to be able to say variable1 = quantity*Rates.amount and return variable1 to the my Index, Delete, Details views. I don't want to store variable1 to my database but i just want to display in my views.....but i don't know how and where to do it

Code from Project material class..

public class ProjectMaterial
{
    public int ProjectMaterialID { get; set; }

    [Required]
    [Display(Name = "Scope Name")]
    public int? ScopeID { get; set; }

    [Required]
    [Display(Name = "Rate Code")]
    public int? RateID { get; set; }

    [Required]
    [Display(Name = "Quantity")]
    public decimal Quantity { get; set; }


    public virtual Scope Scopes { get; set; }
    public virtual Rate Rates { get; set; }

}

Code from scope class..

public class Rate
{
    public int RateID { get; set; }


    [Required]
    [Display(Name = "Rate Code")]
    public int RateCode { get; set; }

    [Required]
    [Display(Name = "Unit")]
    public string Unit { get; set; }

    [Required]
    [Display(Name = "Description")]
    public string Description { get; set; }

    [Required]
    [Display(Name = "Amount")]
    public decimal Amount { get; set; }

    public virtual ICollection<ProjectMaterial> ProjectMaterials { get; set; }
}

Code from project controller class...

public class ProjectMaterialController : Controller
{
    private ContructorContext db = new ContructorContext();

    //
    // GET: /ProjectMaterial/

    public ViewResult Index()
    {
        var projectmaterials = db.ProjectMaterials.Include(p => p.Scopes).Include(p => p.Rates);

        return View(projectmaterials.ToList());
    }



    //
    // GET: /ProjectMaterial/Details/5

    public ViewResult Details(int id)
    {
        ProjectMaterial projectmaterial = db.ProjectMaterials.Find(id);
        return View(projectmaterial);
    }

    //
    // GET: /ProjectMaterial/Create

    public ActionResult Create()
    {
        ViewBag.ScopeID = new SelectList(db.Scopes, "ScopeID", "ScopeName");
        ViewBag.RateID = new SelectList(db.Rates, "RateID", "Unit");
        return View();
    } 

    //
    // POST: /ProjectMaterial/Create

    [HttpPost]
    public ActionResult Create(ProjectMaterial projectmaterial)
    {
        if (ModelState.IsValid)
        {
            db.ProjectMaterials.Add(projectmaterial);
            db.SaveChanges();
            return RedirectToAction("Index");  
        }

        ViewBag.ScopeID = new SelectList(db.Scopes, "ScopeID", "ScopeName", projectmaterial.ScopeID);
        ViewBag.RateID = new SelectList(db.Rates, "RateID", "Unit", projectmaterial.RateID);
        return View(projectmaterial);
    }

    //
    // GET: /ProjectMaterial/Edit/5

    public ActionResult Edit(int id)
    {
        ProjectMaterial projectmaterial = db.ProjectMaterials.Find(id);
        ViewBag.ScopeID = new SelectList(db.Scopes, "ScopeID", "ScopeName", projectmaterial.ScopeID);
        ViewBag.RateID = new SelectList(db.Rates, "RateID", "Unit", projectmaterial.RateID);
        return View(projectmaterial);
    }

    //
    // POST: /ProjectMaterial/Edit/5

    [HttpPost]
    public ActionResult Edit(ProjectMaterial projectmaterial)
    {
        if (ModelState.IsValid)
        {
            db.Entry(projectmaterial).State = EntityState.Modified;
            db.SaveChanges();
            return RedirectToAction("Index");
        }
        ViewBag.ScopeID = new SelectList(db.Scopes, "ScopeID", "ScopeName", projectmaterial.ScopeID);
        ViewBag.RateID = new SelectList(db.Rates, "RateID", "Unit", projectmaterial.RateID);
        return View(projectmaterial);
    }

    //
    // GET: /ProjectMaterial/Delete/5

    public ActionResult Delete(int id)
    {
        ProjectMaterial projectmaterial = db.ProjectMaterials.Find(id);
        return View(projectmaterial);
    }

    //
    // POST: /ProjectMaterial/Delete/5

    [HttpPost, ActionName("Delete")]
    public ActionResult DeleteConfirmed(int id)
    {            
        ProjectMaterial projectmaterial = db.ProjectMaterials.Find(id);
        db.ProjectMaterials.Remove(projectmaterial);
        db.SaveChanges();
        return RedirectToAction("Index");
    }

Thanx in advance guys!! really need your help.

Answer 1

Seeing as you say you're new to MVC, I've given you a few options and explained which is best and why, because it's better to understand now so you don't get in to bad habits, especially if you start building larger projects.

You don't necessarily need to create a variable, because you can do that calculation in your view. Because you are passing the domain model directly to the view you can do (in razor):

@(Model.Quantity * Model.Rates.Amount)

Although this is the easiest option I wouldn't necessarily recommend this as views should be dumb - see ASP.NET MVC: How dumb should my view be?.

Another option is to do the calculation in the controller and pass the value in the ViewBag, e.g.:

public ViewResult Details(int id)
{
    ProjectMaterial projectmaterial = db.ProjectMaterials.Find(id);
    ViewBag.Price = projectmaterial.Quantity * projectmaterial.Rates.Amountl
    return View(projectmaterial);
}

Then you could use it in your view like:

@ViewBag.Price

Again, this is easy but I wouldn't recommend it, as ViewBag isn't strongly typed - see Is using ViewBag in MVC bad?.

You could put a property on your ProjectMaterial class like, which is a neat solution.

public decimal Price
{
    get
    {
        return Quantity * Rates.Amount;
    }
}

However, if Price is a property that is only ever used within your views (ie you just display it) then it probably shouldn't be in your domain model, as your domain model is just that - storing and accessing the raw data.

Maybe the best way is to create a viewmodel specific to your view (see http://stephenwalther.com/blog/archive/2009/04/13/asp.net-mvc-tip-50-ndash-create-view-models.aspx) with a Price propert. This means that the property is only used where it is needed, the domain model remains just that, your view remains dumb and your domain model is not exposed to your view. See Why Two Classes, View Model and Domain Model? also for a good explanation of view models