Where to put viewModel or Domain object validation annotations?

Question

My question

How do I pass UserCreateViewModel from my Create Controller, which means that my check (ModelState.IsValid) will only work on UserCreateViewModel if annotations are defined on it. But I cannot define a DataAnnotation for each of my ViewModels, because it will be a lot of work. Instead, I want to place it on a custom domain model. So, how can I fix the Create method to fix both my work with annotation and using mapper without adding more code to the controller.

// My method of creating a controller

[HttpPost]
public ActionResult Create(UserCreateViewModel user)
{
    if (ModelState.IsValid)
    {
        var createUser = new User();
        Mapper.Map(user, createUser);

        _repository.Add(createUser);

        return RedirectToAction("Details", new { id = createUser.UserId });
    }

    return View("Edit", user);
}

// UserCreateViewModel -> Create a specific view model

public class UserCreateViewModel
{
    public string UserName { get; set; }
    public string Password { get; set; }
}

// User → Domain Object

[MetadataType(typeof(User.UserValidation))]
public partial class User 
{
    private class UserValidation
    {
        [Required(ErrorMessage = "UserName is required.")]
        [StringLength(50, MinimumLength = 2, ErrorMessage = "{0} is between {1} to {2}")]
        [RegularExpression(@"(\S)+", ErrorMessage = "White space is not allowed")]
        public string UserName { get; set; }

        [Required(ErrorMessage = "Password is required.")]
        [StringLength(50, MinimumLength = 2, ErrorMessage = "{0} is between {1} to {2}")]
        public string Password { get; set; }
    }
}  

Answer 1

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