Question

我正在开发一个WPF应用程序，该程序使用Entity Framework 6.2从SQL Server Express数据库获取其数据以访问数据。有人告诉我不要使用寿命长的DbContext，因此我修改了应用程序以使用寿命短的DbCcontext。这意味着我在启动期间仅加载一次数据，然后再处理上下文。对于每个更新/删除/插入操作，我都会创建一个新上下文来保存更改=>短暂上下文。

现在我遇到了一个问题：当我尝试插入一个已经设置了所有导航属性的新实体时，我遇到了异常：

“ City_Houses” AssociationSet中的关系处于“已添加”状态。给定多重性约束，相应的“ City_Houses_Target”也必须处于“已添加”状态。

我写了一个示例代码，它复制了确切的错误消息，在这里您可以参见DbContext和我正在使用的模型类：

public class Context : DbContext
{
    public DbSet<City> dbsCities    { get; set; }
    public DbSet<House> dbsHouses   { get; set; }
    public DbSet<Person> dbsPersons { get; set; }

    public Context() : base(@"Server=.\SQLExpress;Database=test;Trusted_Connection=Yes;Connection Timeout=5")
    {
        Configuration.AutoDetectChangesEnabled = false;
        Configuration.LazyLoadingEnabled = false;
        Configuration.ProxyCreationEnabled = false;
    }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<City>().HasMany(c => c.Houses).WithRequired(c => c.City).WillCascadeOnDelete(true);
        modelBuilder.Entity<House>().HasMany(p => p.Residents).WithRequired(p => p.House).WillCascadeOnDelete(true);
    }
}

public class City
{
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual ICollection<House> Houses { get; set; }

    public City()
    {
        Houses = new List<House>();
    }
}

public class House
{
    public int Id      { get; set; }
    public int Number  { get; set; }

    public virtual City City                     { get; set; }
    public virtual ICollection<Person> Residents { get; set; }

    public House()
    {
        Residents = new List<Person>();
    }
}

public class Person
{
    public int Id           { get; set; }
    public string Firstname { get; set; }
    public string Lastname  { get; set; }

    public virtual House House { get; set; }

    public Person()
    {
    }
}

这是我用来保存更改的方法

private void AddCity(object sender, RoutedEventArgs e)
{
        using (var ctx = new Context())
        {
            ctx.dbsCities.Add(new City() { Name = "New York" });
            ctx.SaveChanges();
        }
}

private void AddHouse(object sender, RoutedEventArgs e)
{
        City c = null;

        using (var ctx = new Context())
        {
            c = ctx.dbsCities.AsNoTracking().FirstOrDefault();
        }

        House h = new House() { Number = 66, City = c };
        c.Houses.Add(h);

        using (var ctx = new Context())
        {
            ctx.dbsHouses.Add(h);
            ctx.Entry(h.City).State = System.Data.Entity.EntityState.Unchanged;
            ctx.SaveChanges();
        }
}

private void AddPeople(object sender, RoutedEventArgs e)
{
        City c = null;

        using (var ctx = new Context())
        {
            c = ctx.dbsCities.Include(x=>x.Houses.Select(h=>h.Residents)).AsNoTracking().FirstOrDefault();
        }

        Person p = new Person() { Firstname = "Max", Lastname = "Power" };
        p.House = c.Houses.First();
        c.Houses.First().Residents.Add(p);

        using (var ctx = new Context())
        {
            ctx.dbsPersons.Add(p);
            ctx.Entry(p.House).State = System.Data.Entity.EntityState.Unchanged;
            ctx.SaveChanges();
        }
}

仅当我致电AddPeople时才会发生错误。我在这里做什么错了？

实体框架保存断开连接的实体

0 个答案: