c# - Fluent API EF 继承和映射

Question

我有一个系统，其中包含多个具有一对多关系(父子关系)的自引用实体。我想为所有这些实体使用公共(public)基类:

public class SelfReferencing
{
  public SelfReferencing Parent {get; set;}
  public ICollection<SelfReferencing> Children {get; set;}
}

并从 SelfReferencing 继承特定的实体。在尝试执行以下操作时，Fluent API 映射要求引用属性为定义类型:

modelBuilder.Entity<ConcreteSelfReferencing>()
                .HasMany(e => e.Children)
                .WithOptional(e => e.Parent);

那么，你能帮我找到一种利用继承并映射实体的可能性吗？

谢谢

Answer 1

Note: The example below is known as Table-Per-Hierarchy (TPH) - i.e. all contained in one table. Click on this link for Table-Per-Type (TPT), which has different tables for each type.

当使用基类型和继承类型时，您必须告诉 EF 如何确定特定继承类型的关联。

获取您的代码:

public abstract class SelfReferencing
{
    public SelfReferencing Parent { get; set; }
    public ICollection<SelfReferencing> Children { get; set; }
}

public class ConcreteSelfReferencing : SelfReferencing
{
}

EF 现在必须确定子类是 ConcreteSelfReferencing 还是任何其他类型的子类。这是由表本身的鉴别器确定的，到该表的列不是映射的一部分。

再举个例子，类似于我过去用过的:

public abstract class Policy
{
   public int Id { get; set; }
   public string PolicyNumber { get; set; }
}

public class InsurancePolicy : Policy
{
}

public class CarPolicy : Policy
{
}

表格的结构是这样的:

| Id    |   PolicyNumber  | Type  |  ..... |
  1         CAR0001         C
  2         ISN0001         I

要让 EF 正确生成它们，您需要:

public class MyContext : DbContext
{
   public MyContext() : base()
   {
   }

   public DbSet<Policy> Policies { get; set; }

   protected override void OnModelCreating(ModelBuilder builder)
   {
      var policyMap = modelBuilder.Entity<Policy>();

      // Set up discriminators
      policyMap.Map<InsurancePolicy>(p => o.Requires("Type").HasValue("I"))
               .Map<CarPolicy>(p => o.Requires("Type").HasValue("C"));

      // Notice that `Type` is only used for the discriminators, not an actual
      // mapped property
      policyMap.HasKey(x=>x.Id);
      policyMap.Property(x=>x.PolicyNumber);
   }
}

从您的代码中，您可以自己进行过滤，或者将过滤放在 DbContext 中。这是来自单独类(class)的示例。

public class PolicyRepository
{
   private MyContext context = new MyContext();

   public PolicyRepository()
   {
   }

   public IQueryable<InsurancePolicy> GetInsurancePolicies()
   {
      return this.context.Policies.OfType<InsurancePolicy>();
   }

   public IQueryable<CarPolicy> GetCarPolicies()
   {
      return this.context.Policies.OfType<CarPolicy>();
   }
}