How to update many to many navigation property only? - asp.net-mvc

Question

i can update some properties use under method
public void Update(T entity, params Expression<Func<T, object>>[] properties)
{
_dbSet.Attach(entity);
DbEntityEntry<T> entry = _context.Entry(entity);
foreach (var selector in properties) { entry.Property(selector).IsModified = true; }
}
//repo.Update(entity, e => e.Name, e => e.Description);
but,i want to update navigation property only in many to many relationship
such like admin role adminrole i want to update adminrole table only

Answer 1

If you want to update just relation you can use simple tricks.
To add a new relation between admin and role you need to do:
Admin admin = new Admin { Id = adminId };
context.Admins.Attach(admin);
Role role = new Role { Id = roleId };
context.Roles.Attach(role);
// Create new relation on attached entities
admin.Roles.Add(role);
context.SaveChanges();
To remove existing relation between admin and role you can try this:
Admin admin = new Admin { Id = adminId };
Role role = new Role { Id = roleId };
// Simulate existing relation on detached entities
admin.Roles.Add(role);
context.Admins.Attach(admin);
// Remove existing relation on attached entities
admin.Roles.Remove(role);
context.SaveChanges();
In both scenarios you just need to know keys for admin and role.
Note: This is for scenario where you have real many-to-many (with junction table containing only keys).