EF7（Core）中同一个表的多个关系

Question

我有这样的模特

public class Question
{
    public string Id { get; set; } = Guid.NewGuid().ToString();

    public Answer Answer { get; set; }
    public List<Variant> Variants { get; set; }

    public string CorrectVariantId { get; set; }
    public Variant CorrectVariant { get; set; }
}

public class Variant
{
    public string Id { get; set; } = Guid.NewGuid().ToString();

    public string QuestionId { get; set; }
    public Question Question { get; set; }
}

// mapping

modelBuilder.Entity<Question>()
    .HasOne(q => q.CorrectVariant)
    .WithOne(v => v.Question)
    .HasForeignKey<Question>(q => q.CorrectVariantId);

modelBuilder.Entity<Variant>()
    .HasOne(v => v.Question)
    .WithMany(a => a.Variants)
    .OnDelete(DeleteBehavior.Cascade);

在我从EF RC1升级到RTM之前，这种方法非常有效。但现在它抛出：System.InvalidOperationException: Cannot create a relationship between 'Question.Variants' and 'Variant.Question', because there already is a relationship between 'Question.CorrectVariant' and 'Variant.Question'. Navigation properties can only participate in a single relationship.

如果不删除Variants模型中的Question属性，是否有解决此问题的方法？

Answer 1

万一有人会遇到这个问题。这是更优雅的解决方案

public class Question
{
    public Guid Id { get; private set; }
    public IReadOnlyList<Variant> Variants { get; private set; }
    public Guid CorrectVariantId { get; private set; }
    public Guid? AnsweredVariantId { get; private set; }    
    public bool IsAnswerCorrect => CorrectVariantId == AnsweredVariantId;
    public bool IsAnswered => AnsweredVariantId != null;
}

public class Variant
{
    public Guid Id { get; private set; }
    public Guid QuestionId { get; private set; }
    public string HiddenUserLogin { get; private set; }
    public User HiddenUser { get; private set; }
}

// mapping
mb.Entity<Question>()
    .HasMany(q => q.Variants)
    .WithOne()
    .HasForeignKey(nameof(Variant.QuestionId))
    .IsRequired()
    .OnDelete(DeleteBehavior.Cascade);

mb.Entity<Question>()
    .HasOne(typeof(Variant))
    .WithOne()
    .HasForeignKey<Question>(nameof(Question.AnsweredVariantId))
    .IsRequired(false) 
    .OnDelete(DeleteBehavior.Restrict);

// EF creates Unique Index for nullable fields
mb.Entity<Question>()
    .HasIndex(q => q.AnsweredVariantId)
    .IsUnique(false);

// create index instead of FK hence the cyclic dependency between Question and Variant
mb.Entity<Question>()
    .HasIndex(q => q.CorrectVariantId)
    .IsUnique();

Answer 2

这是RC1中的一个错误/不受欢迎的行为，它已得到修复。

你应该创建另一个属性，比如说另一个关系的第二个问题。

public class Question
{
  public string Id { get; set; } = Guid.NewGuid().ToString();
  public List<Variant> Variants { get; set; }

  public string CorrectVariantId { get; set; }
  public Variant CorrectVariant { get; set; }
}

public class Variant
{
  public string Id { get; set; } = Guid.NewGuid().ToString();

  public string QuestionId { get; set; }
  public Question Question { get; set; }

  public Question SecondQuestion { get; set; }
}

你的DbContext：

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
  modelBuilder.Entity<Question>()
    .HasOne(q => q.CorrectVariant)
    .WithOne(v => v.SecondQuestion)
    .HasForeignKey<Question>(q => q.CorrectVariantId);

  modelBuilder.Entity<Variant>()
      .HasOne(v => v.Question)
      .WithMany(a => a.Variants).HasForeignKey(x => x.QuestionId).OnDelete(DeleteBehavior.SetNull);

  base.OnModelCreating(modelBuilder);
}

如何使用它：

using (var myDb = new MyDbContext())
{
  var variantFirst = new Variant();
  var variantSecond = new Variant();

  var question = new Question();
  variantFirst.Question = question;

  variantSecond.SecondQuestion = question;

  myDb.Variants.Add(variantFirst);
  myDb.Variants.Add(variantSecond);

  myDb.SaveChanges();
}

Answer 3

给出的两个例子已经让我成为了那里的一部分，但是我想要一个集合和一个具有相同对象类型的项目，因此我的模型上的表格与原始问题中的相同。我试图提供一个适用于.NET Core 2.2的简单示例：

public class ParentModel
{
    public int Id { get; set; }

    // Id for single instance navigation property
    public int? ChildModelId { get; set; }

    // Single instance navigation property to ChildTable, identified by ChildModelId property as foreign key
    public virtual ChildModel ChildModel { get; set; }

    // Collection navigation property to ChildTable with identified by ParentId property
    public virtual ICollection<ChildModel> ChildModels { get; set; }
}

public class ChildModel
{
    public int Id { get; set; }

    // Id for ParentModel property back to ParentTable
    public int ParentId { get; set; }

    // Single instance navigation property to ParentTable, identified by ParentId property as foreign key
    public virtual ParentModel ParentModel { get; set; }
}

public class ApplicationDbContext : IdentityDbContext
{
    public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options)
        : base(options)
    {   
    }

    protected override void OnModelCreating(ModelBuilder builder)
    {
        base.OnModelCreating(builder);

        builder.Entity<ParentModel>()
            .ToTable("ParentTable");

        // Configure collection of ChildModels (ParentTable to ChildTable/one-to-many relationship)
        builder.Entity<ParentModel>()
            .HasMany(t => t.ChildModels)
            .WithOne(t => t.ParentModel)
            .HasForeignKey(t => t.ParentId)
            .IsRequired()
            .OnDelete(DeleteBehavior.Cascade);

        builder.Entity<ChildModel>()
            .ToTable("ChildTable");

        // Configure single ChildModel navigation property on ParentModel (one-to-one relationship)
        builder.Entity<ParentModel>()
            .HasOne(t => t.ChildModel)
            .WithOne()
            .HasForeignKey(typeof(ParentModel), nameof(ParentModel.ChildModelId))
            .IsRequired(false)
            .OnDelete(DeleteBehavior.Restrict);
    }
}

避免Navigation properties can only participate in a single relationship.错误的关键是将导航属性仅配置回父表一次。我们使用ChildModels为ParentTable上的.WithOne(t => t.ParentModel)集合配置此项。然后，我们不会通过调用.WithOne()为空来为后续关系配置关系的另一面，因为如果我们再次配置它（例如.WithOne(t => t.ParentModel)），它将会出错。

此外，导航属性上的virtual修饰符是允许lazy loading。