ASP.net MVC + ASP.net Identity Seeding roles and users

In Migrations Configuration class, there is Seed method, and it looks like:

protected override void Seed(DatabaseContext context)
        {
           var  roleManager = new RoleManager<IdentityRole>(new RoleStore<IdentityRole>(context));
           var  userManager = new UserManager<User>(new UserStore<User>(context)); 

            if (!roleManager.RoleExists("Administrator"))
            {
                roleManager.Create(new IdentityRole("Administrator"));
            }

            var user = new User {UserName = "someUserName"};


            if (userManager.FindByName("someUserName") == null)
            {
                var result = userManager.Create(user, "password");

                if (result.Succeeded)
                {
                    userManager.AddToRole(user.Id, "Administrator");
                }    

            }
            }

And then in PM console I've run update-database, and it successfully seeded database, and in table AspNetUserRoles I can see that user someUserName and Administrator role are connected.

But when I put attribute [Authorize(Roles = "Administrator")] above controller I've been redirected to log in page as if my user wasn't in that role, so I've added line in my controller:

var x = _userManager.IsInRole(_userManager.FindByName("someUserName").Id, "Administrator");

And x is false. Why is this happening ? And how can I fix it ? And if this isn't proper way to seed database, which is ?