EF7（Core）中同一個表的多個關系

Question

我有這樣的模特

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);

在從EF RC1升級到RTM之前，這種方法非常有效。 但現在它拋出： 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. 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.

如果沒有從Question模型中刪除Variants屬性，是否有解決此問題的方法？

Answer 1

萬一有人會遇到這個問題。 這是更優雅的解決方案

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

這是RC1中的一個錯誤/不受歡迎的行為，它已得到修復。

你應該創建另一個屬性，比如說另一個關系的SecondQuestion。

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

給出的兩個例子已經讓我成為了那里的一部分，但是我想要一個集合和一個具有相同對象類型的項目，因此我的模型上的表格與原始問題中的相同。 我試圖提供一個適用於.NET Core 2.2的簡單示例：

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.的關鍵Navigation properties can only participate in a single relationship. 錯誤是僅將導航屬性配置回父表一次。 我們使用.WithOne(t => t.ParentModel)為ParentTable上的ChildModels集合配置此項。 然后，我們不會通過調用.WithOne()為后續關系配置關系的另一面，因為如果我們再次配置它（例如.WithOne(t => t.ParentModel) ），它將會出錯。

此外，導航屬性上的virtual修飾符允許延遲加載 。