EF Code First 4.1 - 如何配置默认的一对多关系

Question

我有一个引用地址集合的客户实体。这里的复杂之处在于我希望能够将特定地址识别为默认地址。

如果可能的话，我想在客户表中保留默认地址的外键。这似乎比在地址表中有一列来标识默认值更优雅。

我在使用 Fluent API 来定义这种关系方面遇到了困难。当我运行以下代码时，我收到一个异常： “保存未公开其关系的外键属性的实体时发生错误。EntityEntries 属性将返回 null，因为无法将单个实体识别为异常源。可以更轻松地处理保存时的异常通过在实体类型中公开外键属性来获取详细信息，请参阅 InnerException。” “无法确定依赖操作的有效顺序。由于外键约束、模型要求或存储生成的值，可能存在依赖关系。”

我创建了一个控制台应用程序来显示精确的问题。在此测试应用程序中，我有一个客户实体、一个地址和 flient api 配置。

任何帮助将不胜感激：

using System;
using System.Collections.Generic;
using System.Data.Entity.ModelConfiguration;
using System.ComponentModel.DataAnnotations;
using System.Data.Entity;

namespace OneToManyWithDefault
{

    public class Customer
    {
        private ICollection<Address> m_Addresses;

        public Customer()
        {
            Addresses = new List<Address>();
        }

        public int Id { get; set; }
        public string CompanyName { get; set; }
        public virtual ICollection<Address> Addresses
        {
            get
            {
                if (m_Addresses == null)
                {
                    m_Addresses = new List<Address>();
                }
                return m_Addresses;
            }
            set
            {
                m_Addresses = value;
            }
        }
        public Address DefaultAddress { get; set; }
        public int DefaultAddressId { get; set; }

    }

    public class Address
    {
        public int Id { get; set; }
        public string Town { get; set; }
        public Customer Customer { get; set; }
    }

    public class MyContext
        : DbContext
    {
        public DbSet<Customer> Customers { get; set; }

        public MyContext(string connectionString)
            : base(connectionString)
        {

        }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Configurations.Add(new CustomerConfiguration());
            modelBuilder.Configurations.Add(new AddressConfiguration());
            base.OnModelCreating(modelBuilder);
        }
    }

    public class CustomerConfiguration
        : EntityTypeConfiguration<Customer>
    {
        public CustomerConfiguration()
            : base()
        {
            HasKey(p => p.Id);
            Property(p => p.Id)
                .HasColumnName("Id")
                .HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity)
                .IsRequired();
            Property(p => p.CompanyName)
                .HasColumnName("Name")
                .IsRequired();

            // Configure the mapping for the Default Address (this is likely to be wrong!):
            HasRequired(p => p.DefaultAddress).WithMany()
                .Map(x => x.MapKey("DefaultAddressId"))
                .WillCascadeOnDelete(false);
            HasRequired(p => p.DefaultAddress)
                .WithMany()
                .HasForeignKey(x => x.DefaultAddressId);

            ToTable("Customers");
        }
    }

    public class AddressConfiguration
        : EntityTypeConfiguration<Address>
    {
        public AddressConfiguration()
            : base()
        {
            HasKey(p => p.Id);
            Property(p => p.Id)
                .HasColumnName("Id")
                .HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity)
                .IsRequired();
            Property(p => p.Town)
                .HasColumnName("Town")
                .IsRequired();

            HasRequired(p => p.Customer)
                .WithMany(c => c.Addresses)
                .Map(x => x.MapKey("CustomerId"));

            ToTable("Addresses");
        }
    }

    class Program
    {
        private const string ConnectionString =
            @"Server=.\sql2005;Database=OneToManyWithDefault;integrated security=SSPI;";

        static void Main(string[] args)
        {
            Customer headOffice = new Customer();
            headOffice.CompanyName = "C1";

            Address address = new Address();
            address.Town = "Colchester";
            headOffice.Addresses.Add(address);

            address = new Address();
            address.Town = "Norwich";
            headOffice.Addresses.Add(address);
            headOffice.DefaultAddress = address;

            MyContext context = new MyContext(ConnectionString);
            context.Customers.Add(headOffice);
            context.SaveChanges();

            Console.WriteLine("Done.");
            Console.ReadLine();
        }
    }
}

非常感谢，

保罗。

Answer 1

我不明白 EF 在异常中所说的“未公开外键”是什么。我认为内部异常是重要的部分：

无法确定有效的排序
对于相关操作。依赖关系
可能由于外键而存在
约束、模型要求或
存储生成的值。

我认为您的模型中的问题在于您在 Customer 和 Address 之间存在相互依赖性：地址需要客户（您已将其标记为必需必需的）。那么，EF 不知道在示例代码中首先保存哪个实体 - 默认地址还是客户？两个实体都需要使用有效的 FK 约束来保存对方的主键。

我能看到的最简单的解决方案是在模型中将默认地址设置为可选，然后保存两次（我忽略了按惯例工作的映射）：

public class Customer
{
    private ICollection<Address> m_Addresses;

    public Customer() { Addresses = new List<Address>(); }

    public int Id { get; set; }
    public string CompanyName { get; set; }
    public virtual ICollection<Address> Addresses { get { ... } set { ... } }
    public Address DefaultAddress { get; set; }
    public int? DefaultAddressId { get; set; } // FK for optional relationship
}

public class Address
{
    public int Id { get; set; }
    public string Town { get; set; }
    public Customer Customer { get; set; }
}

// ...

public class CustomerConfiguration : EntityTypeConfiguration<Customer>
{
    public CustomerConfiguration() : base()
    {
        Property(p => p.CompanyName)
            .HasColumnName("Name")
            .IsRequired();

        HasMany(c => c.Addresses)
            .WithRequired(a => a.Customer)
            .Map(x => x.MapKey("CustomerId"));
    }
}

public class AddressConfiguration : EntityTypeConfiguration<Address>
{
    public AddressConfiguration() : base()
    {
        Property(p => p.Town)
            .HasColumnName("Town")
            .IsRequired();
    }
}

然后您的程序将如下所示：

static void Main(string[] args)
{
    Customer headOffice = new Customer();
    headOffice.CompanyName = "C1";

    Address address = new Address();
    address.Town = "Colchester";
    headOffice.Addresses.Add(address);

    address = new Address();
    address.Town = "Norwich";
    headOffice.Addresses.Add(address);

    //headOffice.DefaultAddress = address;
    //We don't set the default address here as SaveChanges would throw an
    //exception. But because it is optional now we are allowed to leave it null.

    MyContext context = new MyContext(ConnectionString);
    context.Customers.Add(headOffice);
    context.SaveChanges();

    headOffice.DefaultAddress = address; // headoffice and address have now PKs
    context.SaveChanges(); // Updates headoffice in the DB with default address
}

这个双 SaveChanges 很丑陋，但我没有看到其他方法。