Clear ViewModel validation Error - asp.net-mvc-4

Question

I have created a ViewModel which contains other models
namespace CreatingLayout.Models
{
public class ViewModel
{
public tbl_Roles Roles { get; set; }
public tbl_Category Category { get; set; }
public tbl_Course Course { get; set; }
public tbl_User User { get; set; }
}
}
And this sub Model has some Required Field field like tbl_User class has following required fields:
public partial class tbl_User
{
[Required(ErrorMessage = "Please enter {0}")]
public string UserName { get; set; }
[Required(ErrorMessage = "Please enter {0}")]
public string Password{ get; set; }
//And many more with Required Field
}
Now my problem is, in my View i am not passing data for all this required fields but only for few. So once i click to submit button, i am calling my Controller Action Method and i am checking for ModelState errors
public ActionResult AdminInsertion(ViewModel objViewModel)
{
if (ModelState.IsValid)
{
//IF No Errors do something
}
return View();
}
So everytime my Modelstate.Isvalid is returning false due to Validation Error which i gave in those 4 classes i.e., tbl_Roles, tbl_Category, tbl_Course and tbl_User. So what i want is to Clear all those error for which i am not passing Data, something like
//Here i want to go to individual classes ex: item should contains all tbl_Roles column fields
foreach(var item in ModelState.Keys)
{
//Now i want to Get Key items of each individual class which is causing Error
foreach(string Key in item)
{
//Here i am going to clear all those error for which i am not passing data
ModelState[Key].Error.Clear();
}
}
But my problem is, i am not able to access fields of tbl_Roles, tbl_Category, tbl_Course and tbl_User. ,How can clear only selected field errors here?