ASP.NET MVC Custom Validation in View Model Best Practice - asp.net-mvc

Question

I am trying to use a combination of Domain Driven Design with Test Driven Development for this application I am building in ASP.NET MVC 3. My archictecture is set up with Repositories, Domain Models, View Models, Controllers and Views. All validation will be handled in the view model. I set up my view model to inherit from "IValidatableObject" so that my validation attributes and my custom validation that i set up in the "Validate" method are both executed when my controller method calls "ModelState.IsValid". The problem I am running into is accessing my repository in the Validate method of my view model. I need to access the repository to check for duplicate records in the database. It seems like the best idea would be to create a property of IRepository type and set that property by passing injecting my repository into the constructor of the view model. For example:
public class UserViewModel : IValidatableObject
{
public UserViewModel(User user, IUserRepository userRepository)
{
FirstName = user.FirstName;
LastName = user.LastName;
UserRepository = userRepository;
UserName = user.UserName;
}
public string UserName { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public IUserRepository UserRepository { get; set; }
public IEnumerable<ValidationResult> Validate()
{
UserCriteria criteria = new UserCriteria { UserName = this.UserName };
IList<User> users = UserRepository.SearchUsers(criteria);
if (users != null && users.count() > 0)
{
yield return new ValidationResult("User with username " + this.UserName + " already exists."
}
}
}
Do you guys think this is a good idea?

Answer 1

It is good enough but if I were you, I would use
...
private readonly Func<IUserRepository> userRepositoryFactory;
...
public IEnumerable<ValidationResult> Validate()
{
UserCriteria criteria = new UserCriteria { UserName = this.UserName };
using(var UserRepository = userRepositoryFactory())
{
IList<User> users = UserRepository.SearchUsers(criteria);
if (users != null && users.count() > 0)
{
yield return new ValidationResult("User with username " + this.UserName + " already exists."
}
}
}

Answer 2

You can add Domain Service class to get object match with your criteria and validated at domain service level
public class PurchaseOrder
{
public string Id { get; private set; }
public string PONumber { get; private set; }
public string Description { get; private set; }
public decimal Total { get; private set; }
public DateTime SubmissionDate { get; private set; }
public ICollection<Invoice> Invoices { get; private set; }
public decimal InvoiceTotal
{
get { return this.Invoices.Select(x => x.Amount).Sum(); }
}
}
public class PurchaseOrderService
{
public PurchaseOrderService(IPurchaseOrderRepository repository)
{
this.repository = repository;
}
readonly IPurchaseOrderRepository repository;
public void CheckPurchasedOrderExsist(string purchaseOrderId)
{
var purchaseOrder = this.repository.Get(purchaseOrderId);
if (purchaseOrder != null)
throw new Exception("PO already exist!");
}
}