ef6 set only foreign key property of navigation property while saving - entity-framework

Question

When I save entity which has navigation property which I use as foreign key:
this.HasRequired<Role>(c => c.Role).WithMany().Map(c => c.MapKey("role_id"));
I set only foreign key property of this navigation property (I get it from web page) thereby other properties of this navigation property are empty, but they have required restrictions:
this.Property(c => c.RoleName).IsRequired();
It's the reason why I get "dbentityvalidationexception" exception with error "field is required".
Is it possible to solve this problem by somehow?
Or I must get full entity for that navigation property from DB, set navigation property of entity which I save and then save my initial entity (It works now, but it doesn't look like good solution)?
Thanks in advance.
This is MS MVC action where I handle the model:
[HttpPost]
public async Task<ActionResult> AddAsync(Staff staff)
{
await staffService.InsertAsync(staff);
return RedirectToAction("Index");
}
and that part of view where I set the property:
<dt>
#Html.Label("Role")
</dt>
<dd>
#Html.DropDownListFor(_=>_.Role.Id, new SelectList(ViewBag.Roles, "Id", "RoleName"), "- Please select a Role -")
</dd>
this is the type of model "Staff"
public class Staff
{
public Staff()
{
Name = new Name();
}
public int Id { get; set; }
public Name Name { get; set; }
...
public virtual Role Role { get; set; }
...
}

Answer 1

I have found more or less good solution for that problem.
I have set all navigation property for which I have only foreign key property as
EntityState.Unchanged.
Now I have this method for saving only specific entity (Staff)
public virtual Staff InsertStaff(Staff entity)
{
context.Entry(entity.Role).State = EntityState.Unchanged;
context.Entry(entity.Maneger).State = EntityState.Unchanged;
SetStaffNavigationPropertiesUnchanged(entity);
return dbSet.Add(entity);
}
and this for saving full graph (in base generic class):
public virtual TEntity InsertGraph(TEntity entity)
{
return dbSet.Add(entity);
}
using "EntityState.Unchanged" (I will show in simplest way) lets me saving Staff when Role has only foreign key property filled (Role has RoleName required property)
using (ClinchContext context = new ClinchContext())
{
var role = new Role { Id = 1 };
Staff staff = context.Staffs.Add(new Staff
{
Name = new Name { Firstname = "FN", Surname = "S", Patronymic = "P" },
Email = "E",
Login = "L",
IsDeleted = false,
Role = role,
Maneger = null//nullable for now
});
context.Entry(staff.Role).State = EntityState.Unchanged;
context.SaveChanges();
}
If someone has more proper solution I would be happy to know, Thank you.