What is difference between Model and ViewModel in asp.net core mvc?

Question

I have a Account class for account models.

public class Account
{
    [Key]
    public Int64 UID { get; set; }

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

    [Required]
    public string PassWord { get; set; }
    [Required]
    public string UserName { get; set; }

}

My project is not a code first project and this is a model class for database 'Account'.

But I use only two properties in login view, string ID and string PassWord. So I can not use ModelState.Isvalid() in the login controller when I check the validation of model because I use just two properties...

So I searched about that then, now I found about 'ViewModel' which is the model class for view.

Then I created a new class 'AccountViewModel' and then I mapped this with view instead of 'Account' model.

Did my way was right? I understood the ViewModel is a model class just for View. And The model class is for all. (like a global meaning...? for DB,view and so on)

What is different between Model and ViewModel class? May I get some a nice way to solve this?

Answer 1

As the name says, view model is very specific to the view.It will be a simple POCO with only those properties needed for the view.

Your other model class is your entity models. So if you are using EF code first approach, you need entity class definitions from which EF will generate the database tables. So basically these entity classes look very similar to your db schema structure.

By creating a view model, you are removing the strong coupling of your entity classes to the UI layer. Now your UI layer is independent of your entity classes and if you ever decide to change the data access code from EF to something else, you do not need to touch the views at all.You simply need to update the mapping part(from the view model to the data access/service layer entities)

View models sometimes looks very similar to your entity models, especially if your entity model is a simple table/class.

In your case, since your view is passing a userid and password, you need a simple view model which has only those 2 properties. When user submits the form,you can read the values and use it to build an domain entity class object as needed.

public class LoginViewModel
{
  public string UserId { set;get;}
  public string Password  { set;get; }
}

You can use data annotations with the view models. The MVC model validation framework these data annotations to do the validations. For example, since user should enter a UserId and Password, you may decorate them with appropriate annotations.

public class LoginViewModel
{
   [Required]
   public string UserId { set;get;}

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

The [Key] attribute is more useful when you define an entity class. So i would not think it is needed for a view model. Remember view model is more like a UI concern. It has no idea about your underlying data storage mechanism at all.

Some of the most used attributes with view model properties are

1. Required
2. MinLength
3. Range
4. Url
5. Phone
6. StringLength
7. DataType