public DbController(PawnShopDbContext db, UserManager <User> userManager, RoleManager <Role> roleManager, DatabaseMigrator migrator)
 {
     _roleManager = roleManager;
     _db          = db.GuardIsNotNull(nameof(db));
     _userManager = userManager.GuardIsNotNull(nameof(userManager));
     _roleManager = roleManager.GuardIsNotNull(nameof(roleManager));
     _migrator    = migrator.GuardIsNotNull(nameof(migrator));
 }