how do View Models link to the Database? - asp.net-mvc

Question

I am new to Asp.net MVC and could really use some clarification on how View models work.
From my understanding, View models are used to only expose necessary fields from the domain model to the Views. What I am finding hard to understand is that domain models are linked to the Db via Dbset. So it makes sense to me that when data is posted to a controller using a domain model, that this data can find its way into the Db.
From the examples of View models I have seen, they are not referenced by a Dbset. So how does data posted to a View model find its way into the database. Does EF just match the fields from the View model to fields which match from the domain model?
thanks for your help

Answer 1

As Jonathan stated, AutoMapper will help you map your ViewModel entities to your Domain model. Here is an example:
In your view you work with the View Model (CreateGroupVM):
#model X.X.Areas.Group.Models.CreateGroupVM
#using (Html.BeginForm(null,null, FormMethod.Post, new { #class="form-horizontal", role="form"}))
{
#Html.ValidationSummary()
#Html.AntiForgeryToken()
#Html.LabelFor(model => model.Title, new { #class = "col-lg-4 control-label" })
#Html.TextBoxFor(model => model.Title, new { #class = "form-control" })
#Html.ValidationMessageFor(model => model.Title)
#Html.LabelFor(model => model.Description, new { #class = "col-lg-4 control-label" })
#Html.TextBoxFor(model => model.Description, new { #class = "form-control" })
#Html.ValidationMessageFor(model => model.Description)
#Html.LabelFor(model => model.CategoryId, new { #class = "col-lg-4 control-label" })
#Html.DropDownListFor(x => x.CategoryId, Model.Categories)
#Html.ValidationMessageFor(model => model.CategoryId)
<div class="form-group">
<div class="col-lg-offset-4 col-lg-8">
<button type="submit" class="btn-u btn-u-blue">Create</button>
</div>
</div>
}
ViewModel (CreateGroupVM.cs):
Notice how we pass in a list of Categories - you could not do this had you strictly used your domain model because you cant pass a list of categories in the Group model. This gives us strongly typed helpers in our views, and no ViewBag usage.
public class CreateGroupVM
{
[Required]
public string Title { get; set; }
public string Description { get; set; }
[DisplayName("Category")]
public int CategoryId { get; set; }
public IEnumerable<SelectListItem> Categories { get; set; }
}
Domain Model (Group.cs):
public class Group
{
public int Id { get; set; }
public string Title { get; set; }
public string Description { get; set; }
public int CategoryId { get; set; }
public int CreatorUserId { get; set; }
public bool Deleted { get; set; }
}
In your HttpPost Create Action - you let AutoMapper do the mapping then save to the DB. Note that by default AutoMapper will map fields that are the same name. You can read https://github.com/AutoMapper/AutoMapper/wiki/Getting-started to get started with AutoMapper.
[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Create(CreateGroupVM vm)
{
if (ModelState.IsValid)
{
var group = new InterestGroup();
Mapper.Map(vm, group); // Let AutoMapper do the work
db.Groups.Add(group);
db.SaveChanges();
return RedirectToAction("Index");
}
return View(vm);
}

Answer 2

The view models are in no way tied to your database. You would need to create a new domain model and populate it with the data from the view model in order to save it to the database. Of course, having to do that is very annoying and someone created AutoMapper to handle that.
With automapper you could just match the properties from your view models to properties in the domain model and then add them to the database as needed.