Controller.ValidateModel. How it works?

I have the following model class:

public abstract class CompanyFormViewModelBase
{
    public CompanyFormViewModelBase()
    {
        Role = new CompanyRoleListViewModel();
        ContactPerson = new PersonListViewModel();
        Sector = new SectorListViewModel();
    }
    [Required]
    [Display(Name = "Company Name")]
    public string CompanyName { get; set; }
    public CompanyRoleListViewModel Role { get; set; }
    [Display(Name = "Contact Name")]
    public PersonListViewModel ContactPerson { get; set; }
    public SectorListViewModel Sector { get; set; }
}
public class AddCompanyViewModel : CompanyFormViewModelBase, IValidatableObject
{
    public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
    {
        PlugandabandonEntities db = new PlugandabandonEntities();
        CompanyName = CompanyName.Trim();

        var results = new List<ValidationResult>();

        if (db.Company.Where(p => p.CompanyName.ToLower() == CompanyName.ToLower()).Count() > 0)
            results.Add(new ValidationResult("Company already exists.", new string[] { "CompanyName" }));
        return results;
    }
}

      

        
        
        
      

    

It works great with "classic" using:

    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Create(Plugandabandon.ViewModels.AddCompanyViewModel model)
    {
        if (ModelState.IsValid)
        {
            CreateCompany(model);
            return RedirectToAction("Index");
        }
        else
        {
            return View(model);
        }
    }

      

        
        
        
      

    

But I want to use this model class for another ajax form. I have the following method:

   public JsonResult ReturnJsonAddingCompany(string companyName, int roleID, int sectorID, int personID)
    {
        Plugandabandon.ViewModels.AddCompanyViewModel model = new ViewModels.AddCompanyViewModel()
        {
            CompanyName = companyName,
            ContactPerson = new ViewModels.PersonListViewModel()
            {
                SelectedItem = personID
            },
            Role = new ViewModels.CompanyRoleListViewModel()
            {
                SelectedItem = roleID
            },
            Sector = new ViewModels.SectorListViewModel()
            {
                SelectedItem = sectorID
            }
        };

        ValidateModel(model);

        if (ModelState.IsValid)
        {
            CreateCompany(model);
        }
        else
        {
            throw new Exception("Company with such name already exists");
        }

        var list = Utils.CompanyList();
        return Json(list, JsonRequestBehavior.AllowGet);
    }

      

        
        
        
      

    

Take a look

    ValidateModel(model);

      

        
        
        
      

    

lines. If the model is correct, it works fine. If not correct, it throws an exception and interrupts the continuation of the method (and returns an exception for viewing). Also, if I set a breakpoint to

public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)

      

        
        
        
      

    

it was never called in an invalid model case! (with a valid model the Validate method is called). I want the behavior to be similar to the "classic" method, just check the model and then check ModelState.IsValid. The behavior of ValidateModel (model) is very strange for me, it is a "black box" ...