找到下面描述的方案的解决方案花了我太长时间。看似简单的事情应该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（在我的例子中为Asp.Net）中为现有的“多对一”关系分配不同的结尾。
型号：
我意识到使用ForeignKey关系而不是独立关联可能是一种选择，但我喜欢在我的Pocos中没有ForeignKey实现。
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; }
}
}
目标公司有一位客户：
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; }
}
客户来自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类提供了一些常见属性：
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; }
}
上下文：
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;
}
}
答案 0 :(得分：5)
我等待@Martin的答案，因为有更多解决方案可以解决这个问题。这是另一个（至少它与ObjectContext API一起工作，所以它也应该与DbContext API一起使用）：
// 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。具有强制关系的未更改或已修改状态的实体（在多方面）在没有其他处于已删除状态时不能在添加状态中具有独立关联。根本不允许修改关联状态。
答案 1 :(得分：4)
有很多关于这个主题的信息;在stackoverflow上，我发现Ladislav Mrnka的见解特别有用。有关此主题的更多信息，请访问：NTier Improvements for Entity Framework和此处What's new in Entity Framework 4?
在我的项目（Asp.Net Webforms）中，用户可以选择使用不同的（现有）Customer对象替换分配给Target对象的Customer。此事务由绑定到ObjectDataSource的FormView控件执行。 ObjectDataSource与项目的BusinessLogic层进行通信，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Target对象的过程更加容易。 DbContext正确识别独立关联的客户端，并将更改提交到数据库，而无需进一步操作。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。如果您发现上述方法存在问题，请在评论中告知我们。谢谢！