实体框架，代码优先，更新与独立关联的“一对多”关系

Question

我花了很长时间才找到下面描述的场景的解决方案。看似简单的事情，却被证明是相当困难的。问题是：

使用 Entity Framework 4.1（代码优先方法）和“独立关联”如何在“分离”场景（在我的情况下为 Asp.Net）中为现有的“多对一”关系分配不同的端。

型号：

我意识到使用 ForeignKey 关系而不是独立关联是一种选择，但我更喜欢在我的 Pocos 中没有 ForeignKey 实现。

一位客户有一个或多个目标：

    public class Customer:Person
{
    public string Number { get; set; }
    public string NameContactPerson { get; set; }
    private ICollection<Target> _targets;

    // Independent Association
    public virtual ICollection<Target> Targets
    {
        get { return _targets ?? (_targets = new Collection<Target>()); }
        set { _targets = value; }
    }
}

一个目标有一个客户：

    public class Target:EntityBase
{
    public string Name { get; set; }
    public string Description { get; set; }
    public string Note { get; set; }
    public virtual Address Address { get; set; }
    public virtual Customer Customer { get; set; }
}

Customer 派生自 Person 类：

    public class Person:EntityBase
{        
    public string Salutation { get; set; }
    public string Title { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set  ; }        
    public string Telephone1 { get; set; }
    public string Telephone2 { get; set; }
    public string Email { get; set; }        

    public virtual Address Address { get; set; }
}

EntityBase 类提供了一些通用属性：

    public abstract class EntityBase : INotifyPropertyChanged
{
    public EntityBase()
    {
        CreateDate = DateTime.Now;
        ChangeDate = CreateDate;
        CreateUser = HttpContext.Current.User.Identity.Name;
        ChangeUser = CreateUser;
        PropertyChanged += EntityBase_PropertyChanged;
    }

    public void EntityBase_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        if (Id != new Guid())
        {
            ChangeDate = DateTime.Now;
            ChangeUser = HttpContext.Current.User.Identity.Name;
        }
    }

    protected virtual void OnPropertyChanged(PropertyChangedEventArgs e)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null) handler(this, e);
    }

    public event PropertyChangedEventHandler PropertyChanged;

    public Guid Id { get; set; }
    public DateTime CreateDate { get; set; }
    public DateTime? ChangeDate { get; set; }
    public string CreateUser { get; set; }
    public string ChangeUser { get; set; }
}

背景：

    public class TgrDbContext : DbContext
{
    public DbSet<Person> Persons { get; set; }
    public DbSet<Address> Addresses { get; set; }
    public DbSet<Customer> Customers { get; set; }
    public DbSet<Target> Targets { get; set; }
    public DbSet<ReportRequest> ReportRequests { get; set; }

    // If OnModelCreating becomes to big, use "Model Configuration Classes"
    //(derived from EntityTypeConfiguration) instead
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Person>().HasOptional(e => e.Address);            
        modelBuilder.Entity<Customer>().HasMany(c => c.Targets).WithRequired(t => t.Customer);            
    }

    public static ObjectContext TgrObjectContext(TgrDbContext tgrDbContext)
    {           
        return ((IObjectContextAdapter)tgrDbContext).ObjectContext;
    }
}

Answer 1

我等待@Martin 的回答，因为这个问题有更多的解决方案。这是另一个（至少它适用于 ObjectContext API，因此它也应该适用于 DbContext API）：

// Existing customer
var customer = new Customer() { Id = customerId };
// Another existing customer
var customer2 = new Customer() { Id = customerId2 };

var target = new Target { ID = oldTargetId };
// Make connection between target and old customer
target.Customer = customer;

// Attach target with old customer
context.Targets.Attach(target);
// Attach second customer
context.Customers.Attach(customer2);
// Set customer to a new value on attached object (it will delete old relation and add new one)
target.Customer = customer2;

// Change target's state to Modified
context.Entry(target).State = EntityState.Modified;
context.SaveChanges();

这里的问题是 EF 内部的状态模型和状态验证。当没有其他处于删除状态时，具有强制关系（在多方面）的处于未更改或已修改状态的实体在添加状态下不能具有独立关联。根本不允许修改关联状态。

Answer 2

关于这个主题有很多信息可以找到；在 stackoverflow 上，我发现 Ladislav Mrnka 的见解特别有用。更多关于这个主题的信息也可以在这里找到：NTier Improvements for Entity Framework 和这里What's new in Entity Framework 4?

在我的项目（Asp.Net Webforms）中，用户可以选择用不同的（现有）客户对象替换分配给目标对象的客户。此事务由绑定到 ObjectDataSource 的 FormView 控件执行。 ObjectDataSource 与项目的 BusinessLogic 层通信，后者又将事务传递给 DataAccess 层中 Target 对象的存储库类。存储库类中 Target 对象的 Update 方法如下所示：

    public void UpdateTarget(Target target, Target origTarget)
    {
        try
        {
            // It is not possible to handle updating one to many relationships (i.e. assign a 
            // different Customer to a Target) with "Independent Associations" in Code First.
            // (It is possible when using "ForeignKey Associations" instead of "Independent 
            // Associations" but this brings about a different set of problems.)
            // In order to update one to many relationships formed by "Independent Associations"
            // it is necessary to resort to using the ObjectContext class (derived from an 
            // instance of DbContext) and 'manually' update the relationship between Target and Customer. 

            // Get ObjectContext from DbContext - ((IObjectContextAdapter)tgrDbContext).ObjectContext;
            ObjectContext tgrObjectContext = TgrDbContext.TgrObjectContext(_tgrDbContext);

            // Attach the original origTarget and update it with the current values contained in target
            // This does NOT update changes that occurred in an "Independent Association"; if target
            // has a different Customer assigned than origTarget this will go unrecognized
            tgrObjectContext.AttachTo("Targets", origTarget);
            tgrObjectContext.ApplyCurrentValues("Targets", target);

            // This will take care of changes in an "Independent Association". A Customer has many
            // Targets but any Target has exactly one Customer. Therefore the order of the two
            // ChangeRelationshipState statements is important: Delete has to occur first, otherwise
            // Target would have temporarily two Customers assigned.
            tgrObjectContext.ObjectStateManager.ChangeRelationshipState(
                origTarget,
                origTarget.Customer,
                o => o.Customer,
                EntityState.Deleted);

            tgrObjectContext.ObjectStateManager.ChangeRelationshipState(
                origTarget,
                target.Customer,
                o => o.Customer,
                EntityState.Added);

            // Commit
            tgrObjectContext.Refresh(RefreshMode.ClientWins, origTarget);
            tgrObjectContext.SaveChanges();
        }
        catch (Exception)
        {
            throw;
        }
    }            

这适用于 Target 对象的 Update 方法。值得注意的是，插入新目标对象的过程要容易得多。 DbContext 正确识别独立关联的客户端，并毫不费力地将更改提交到数据库。存储库类中的 Insert 方法如下所示：

        public void InsertTarget(Target target)
    {
        try
        {
            _tgrDbContext.Targets.Add(target);
            _tgrDbContext.SaveChanges();
        }
        catch (Exception)
        {
            throw;
        }
    }

希望这对处理类似任务的人有用。如果您发现上述这种方法存在问题，请在您的 cmets 中告诉我。谢谢！