2

我一直在开发一个数据库的框架，它将支持在 EF 4.3.1 中使用 Code First 对数据进行版本控制。

几天前，我让模型持续存在并正确加载，但从那以后我破坏了一些东西，我不知道出了什么问题。所有的类都被映射并创建了表，数据也被持久化了。所以在存储方向上，一切正常！但是，当我尝试加载Registration实体时，这些值都是默认构造函数设置的值。我在想也许在Registration调用构造函数后数据没有被加载，但我现在已经无法弄清楚发生了什么！

基础是这两个类，我的verion-able类派生自...

public abstract class VersionBase<T> {
    [Key]
    public Int64 Id { get; protected set; }
    public DateTime CreationDateTime { get; protected set; }

    // Value is virtual to support overriding to let deriving classes specify attributes for the property, such as [Required] to specify a non-nullable System.String
    public virtual T Value { get; internal set; }

    protected VersionBase() {
        CreationDateTime = DateTime.Now;
    }

    protected VersionBase(T value)
        : this() {
        Value = value;
    }
}

public abstract class VersionedBase<TVersion, TBase>
    where TVersion : VersionBase<TBase>, new() {
    [Key]
    public Int64 Id { get; protected set; }
    public virtual ICollection<TVersion> Versions { get; protected set; }

    protected VersionedBase() {
        Versions = new List<TVersion>();
    }

    [NotMapped]
    public Boolean HasValue {
        get {
            return Versions.Any();
        }
    }

    [NotMapped]
    public TBase Value {
        get {
            if (HasValue)
                return Versions.OrderByDescending(x => x.CreationDateTime).First().Value;
            throw new InvalidOperationException(this.GetType().Name + " has no value");
        }
        set {
            Versions.Add(new TVersion { Value = value });
        }
    }
}

派生类的例子...

public class VersionedInt32 : VersionedBase<VersionedInt32Version, Int32> { }

public class VersionedInt32Version : VersionBase<Int32> {
    public VersionedInt32Version() : base() { }
    public VersionedInt32Version(Int32 value) : base(value) { }
    public static implicit operator VersionedInt32Version(Int32 value) {
        return new VersionedInt32Version { Value = value };
    }
}

...和...

public class VersionedString : VersionedBase<VersionedStringVersion, String> { }

public class VersionedStringVersion : VersionBase<String> {
    public VersionedStringVersion() : base() { }
    public VersionedStringVersion(String value) : base(value) { }
    public static implicit operator VersionedStringVersion(String value) {
        return new VersionedStringVersion { Value = value };
    }

    /// <summary>
    /// The [Required] attribute tells Entity Framework that we want this column to be non-nullable
    /// </summary>
    [Required]
    public override String Value { get; internal set; }
}

我的调用代码是这样的......

static void Main(String[] args) {
    using (var db = new VersionedFieldsContext()) {
        Registration registration = new Registration();
        registration.FirstName.Value = "Test";
        registration.FirstName.Versions.Add("Derp");
        db.Registration.Add(registration);
        db.SaveChanges();
    }
    using (var db = new VersionedFieldsContext()) {
        Registration registration = db.Registration.First();
        // InvalidOperationException at next line: "VersionedString has no value"
        String asdf = registration.FirstName.Value;
    }
}

public class Registration {
    [Key]
    public Int64 Id { get; set; }
    public DateTime CreationDateTime { get; set; }
    public VersionedString FirstName { get; set; }

    public Registration() {
        CreationDateTime = DateTime.Now;
        FirstName = new VersionedString();
    }
}

public class VersionedFieldsContext : DbContext {
    public DbSet<Registration> Registration { get; set; }

    public VersionedFieldsContext() {
        Database.SetInitializer<VersionedFieldsContext>(new DropCreateDatabaseIfModelChanges<VersionedFieldsContext>());
    }

    protected override void OnModelCreating(DbModelBuilder modelBuilder) {
        base.OnModelCreating(modelBuilder);
        modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();
    }
}

感谢您的任何见解！

4

1 回答 1

2

您需要进行两项更改：

  • FirstName从构造函数中删除实例化，Registration以便构造函数只有：

    public Registration() {
    CreationDateTime = DateTime.Now;
}

    在实体的默认构造函数中创建导航引用（不是集合）的实例会导致已知问题：什么会导致实体框架在现有数据上保存未加载（但可延迟加载）的引用？

  • 如果您已修复第一点，您的自定义异常将更改为NullReferenceException. 要解决此问题，请使用FirstName属性，Registration virtual因为第二个using块中的代码需要延迟加载：

    public virtual VersionedString FirstName { get; set; }

编辑

创建注册并自动实例化的解决方法FirstName可能是工厂方法：

public class Registration {
    [Key]
    public Int64 Id { get; set; }
    public DateTime CreationDateTime { get; set; }
    public VersionedString FirstName { get; set; }

    public Registration() {
        CreationDateTime = DateTime.Now;
    }

    public static Registration Create() {
        return new Registration {
            FirstName = new VersionedString()
        }
    }
}

EF 在具体化对象时使用默认构造Registration函数。在您的自定义代码中，您可以在需要创建以下实例时使用工厂方法Registration：

var registration = Registration.Create();

当您使用更改跟踪或延迟加载代理并希望手动创建代理实例时，它可能不太有用：

var registration = db.Registration.Create();

这将再次调用默认构造函数，您必须在FirstName创建对象后实例化。

于 2012-04-19T16:33:11.540 回答