Asp.net身份2：无法删除或添加用户角色

Question

我使用带有owin Identity 2.1的web api 2.2和实体框架6.1.2一切正常，除非我试图删除或添加角色给用户，我收到以下错误消息：< / p>

操作失败：无法更改关系，因为一个或多个外键属性不可为空。当对关系进行更改时，相关的外键属性将设置为空值。如果外键不支持空值，则必须定义新关系，必须为外键属性分配另一个非空值，或者必须删除不相关的对象。

 if (!await _userManager.IsInRoleAsync(user.Id, role.Name))
         //Excption is thrown here with the above message
         await _userManager.AddToRoleAsync(user.Id, role.Name);

更新1

用于DbContext

public class EFDbContext : IdentityDbContext<ApplicationUser>
    {
        public EFDbContext()
            : base("name=AppConnectionString", throwIfV1Schema: false)
        {
            this.Configuration.LazyLoadingEnabled = false;
        }

        public static EFDbContext Create()
        {
            return new EFDbContext();
        }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            var typesToRegister = Assembly.GetExecutingAssembly().GetTypes()
            .Where(type => !String.IsNullOrEmpty(type.Namespace))
            .Where(type => type.BaseType != null && type.BaseType.IsGenericType && type.BaseType.GetGenericTypeDefinition() == typeof(EntityTypeConfiguration<>));
            Parallel.ForEach(typesToRegister, type =>
            {
                dynamic configurationInstance = Activator.CreateInstance(type);
                modelBuilder.Configurations.Add(configurationInstance);
            });
            base.OnModelCreating(modelBuilder);
        }

        public virtual TEntity AttachEntityToContext<TEntity>(TEntity entity) where TEntity : BaseEntity
        {
            var alreadyAttached = Set<TEntity>().Local.FirstOrDefault(x => x.Id == entity.Id);
            if (alreadyAttached == null)
            {
                Set<TEntity>().Attach(entity);
                return entity;
            }
            else
                return alreadyAttached;
        }

        protected override void Dispose(bool disposing)
        {
            base.Dispose(disposing);
        }

       public new IDbSet<TEntity> Set<TEntity>() where TEntity : BaseEntity
        {
            return base.Set<TEntity>();
        }
    }
}