entity-framework – Entity Framework EntityType’UserAccount

One or more validation errors were detected during model generation:   
tSystem.Data.Entity.Edm.EdmEntityType: : EntityType 'UserAccount' has no key defined. Define the key for this EntityType.
tSystem.Data.Entity.Edm.EdmEntitySet: EntityType: EntitySet 'UserAccounts' is based on type 'UserAccount' that has no keys defined.

此错误由代码触发：

_Db.Database.Initialize(true);

我假设由于某种原因它没有在模型上获取[Key]属性.最初当我尝试运行代码时,我没有添加key属性,这是否意味着已经创建/缓存了某些东西,这阻止了这个密钥的应用？

除了包含Entity Framework 5之外,MVC4项目几乎是一个空白的设置

型号代码

public class AccountContext: DbContext
{
    public AccountContext()
        : base("DefaultConnection")
    {          
    }

    public DbSet<UserAccount> UserAccounts { get; set; }
}

[Table("UserAccount")]
public class UserAccount
{
    [Key]
    [Required]        
    public string Username;

    [Required]
    [DataType(DataType.Password)]
    [StringLength(100,ErrorMessage = "The {0} must be at least {2} characters long.",MinimumLength = 6)]
    public string Password;  

    public string Name;
    public string Surname;

    [Required] 
    public string Email;             
}

Global.asax初始化

Database.SetInitializer(new DropCreateDatabaseAlways<AccountContext>());
var _Db = new AccountContext();
_Db.Database.Initialize(true);

我已经做了一些搜索并理解命名约定,例如Id / UserId等但是我想明确使用[Key]并调用字段Username.