Question

我一直在开发数据库的骨架，该数据库支持使用EF 4.3.1中的Code First对数据进行版本控制。

几天前我让模型坚持并正确加载，但我已经破坏了一些东西，我无法弄清楚出了什么问题。所有类都被映射并创建表，数据也被持久化。所以在storring方向，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将它们设置为的值。我想在调用Registration构造函数后可能没有加载数据，但是我现在能够找出发生了什么的能力！

基础是这两个类，我的可用类派生出来......

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; }
}

我的通话代码就是这样......

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！

Answer 1

您需要进行两项更改：

从FirstName构造函数中删除Registration的实例化，以便构造函数只是：
```
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：What would cause the Entity Framework to save an unloaded (but lazy loadable) reference over existing data?
如果您已修复第一个点，则您的自定义例外会更改为NullReferenceException。要解决此问题，请在FirstName Registration中生成virtual属性，因为第二个using块中的代码需要延迟加载：
```
public virtual VersionedString FirstName { get; set; }
```

修改

创建注册并自动实例化FirstName的解决方法可能是工厂方法：

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在实现Registration对象时使用默认构造函数。在您的自定义代码中，您可以在需要创建Registration的实例时使用工厂方法：

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，它可能没那么有用：

var registration = db.Registration.Create();

这将再次调用默认构造函数，并且必须在已创建对象后实例化FirstName。

Code First导航属性被保留但未加载

1 个答案: