Correctly using a view model

Question

User1352447851 posted

I am trying to use a view model so I am not exposing the model which is modifying the database. In the below code, UserObject is the model with private values that updates the database. UserView is a reduced model which I want to use to display a list on my razor page.

public class UserViewModel
{

public UserViewModel()
{

}

public UserViewModel(UserObject userobject)
{
    DisplayName = userobject.DisplayName;

}

[Required]
public string DisplayName { get; set;}
}

My PageModel:

private readonly MyContext _context;

public MyModel(MyContext context)
{
_context = context;

}

public IList<UserObject> UserObject { get;set; }

public IList<UserViewModel> UserViewModel { get; set; }

public async Task OnGetAsync()
{
    UserObject = await _context.UserObject.Where(x => x.Name == "Bob").ToListAsync(); //needs changing to return IList<UserViewModel>

Can anyone confirm that this is the correct use of a ViewModel?

What syntax is best for convert my UserObjects into UserViewModels in the async list? I can only think to do a long for loop. 

Answer 1

User475983607 posted

Can anyone confirm that this is the correct use of a ViewModel?

ASP.NET Core Razor pages have a built in model.  It's called the PageModel.  I recommend using the PageModel first rather than building a ViewModel.  There can be times when a ViewModel is needed.

Razor pages solve a particular MVC design redundancy.  In MVC you always end up with a GET and POST action with the same name and at least one ViewModel.  Often there are two ViewModels; one populates the UI, the other is submitted to a POST action.  Razor Page combines these several code bits into one Razor Page.

The following site is very informative and I recommend that bookmark it; https://www.learnrazorpages.com/

The PageModel is described at the following link; https://www.learnrazorpages.com/razor-pages/pagemodel

Answer 2

User1352447851 posted

I'm sorry, I don't really understand your answer.

I have a page model (as above), a data model, and I want to add a view model to populate the UI.

I needed to do this perviously on another page as I was submitting a form and as I was originally only using a data model (UserObject). Model.IsValid failed because the client didn't fully populate it all its properties. I created a view model just to handle the form and used my data model to submit to the database:

if (!ModelState.IsValid)
{
return Page();
}

UserObject userobject = new UserObject(UserFormViewModel, "bob"); //this sets some other values in its constructor

_context.UserObject.Add(userobject);
await _context.SaveChangesAsync();

Where UserObject is my data model and UserFormViewModel is my ViewModel.

After looking into that I read that a separate View Model should also be created for sending elements to the UI as well, so as not to expose the complete data model.

Answer 3

User475983607 posted

I think you are looking for a projection query.

List<UserViewModel> userVm = await _context.UserObject.Where(x => x.Name == "Bob").Select(vm => new UserViewModel() {
	DisplayName = vm.Field<string>("DisplayName")
}).ToListAsync(); 

Or

UserViewModel userVm = await _context.UserObject.Where(x => x.Name == "Bob").Select(vm => new UserViewModel() {
	DisplayName = vm.Field<string>("DisplayName")
}).FirstOrDefaultAsync(); 

https://docs.microsoft.com/en-us/dotnet/framework/data/adonet/method-based-query-syntax-examples-projection

Answer 4

User1352447851 posted

Awesome, Thanks