EF Core Annotation for OnDelete

Question

I hava an annotation problem:

        modelBuilder.Entity<FirstClass>()
            .HasOne(f => f.SecondClass)
            .WithOne(s => s.FirstClass)
            .HasForeignKey<FirstClass>(f => f.SecondClassId)
            .OnDelete(DeleteBehavior.Cascade);

How to write this with annotations? I don't find the annotation for OnDelete.

Answer 1

Try this:

Your model

public class FirstClass
    {
        [Key]
        public int Id { get; set; } 
        public int SecondClassId { get; set; }

        [ForeignKey(nameof(SecondClassId))]
        [InverseProperty("FirstClasses")]
        public virtual SecondClass SecondClass { get; set; }
    }
    public class SecondClass
    {
        [Key]
        public int Id { get; set; }
        [InverseProperty(nameof(FirstClass.SecondClass))]
        public virtual ICollection<FirstClass> FirstClasses { get; set; }

    }

if you want to have only one first and one second try this code. But I don't recommend it since it will be hard to find the errors.

public class FirstClass
    {
        [Key]
        public int Id { get; set; } 
        public int SecondClassId { get; set; }

        [ForeignKey(nameof(SecondClassId))]
        [InverseProperty("FirstClass")]
        public virtual SecondClass SecondClass { get; set; }
    }
    public class SecondClass
    {
        [Key]
        public int Id { get; set; }
        [InverseProperty(nameof(FirstClass.SecondClass))]
        public virtual FirstClass FirstClass { get; set; }

    }

Your db context:

public class FirstClassDbContext : DbContext
    {
        public FirstClassDbContext()
        {
        }

        public FirstClassDbContext(DbContextOptions<FirstClassDbContext> options)
            : base(options)
        {
        }
        
        public DbSet<FirstClass> FirstClasses { get; set; }
        public DbSet<SecondClass> SecondClasses { get; set; }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {

            optionsBuilder.UseSqlServer(@"Server=localhost;Database=FirstClass;Trusted_Connection=True;");
        }
        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<FirstClass>(entity =>
            {
                entity.HasOne(d => d.SecondClass)
                   .WithMany(p => p.FirstClasses)
                   .HasForeignKey(d => d.SecondClassId)
                   .OnDelete(DeleteBehavior.ClientSetNull)
                   .HasConstraintName("FK_FirstClass_SecondClass");
            });

          // or for one to one you can use yours, but I don't recommend it since
//it will be hard to find the errors and it will not do anything for you. 
//Only confusing queries.

             modelBuilder.Entity<FirstClass>()
            .HasOne(f => f.SecondClass)
            .WithOne(s => s.FirstClass)
            .HasForeignKey(d => d.SecondClassId)
            .OnDelete(DeleteBehavior.ClientSetNull)
            .HasConstraintName("FK_FirstClass_SecondClass");
        }
    }

Answer 2

I think it is not necessary.

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

    public class FirstClass : GeneralClass
    {
        public int? DerivedClassId { get; set; }
        public string Name { get; set; }
        [InverseProperty(nameof(EFCoreTest.SecondClass.FirstClass))]
        public SecondClass SecondClass { get; set; }
        public DerivedClass DerivedClass {get; set; }
    }

    public class SecondClass : GeneralClass
    {
        public int FirstClassId { get; set; }
        public string Url { get; set; }
        [ForeignKey(nameof(FirstClassId))]
        public FirstClass FirstClass { get; set; }
    }
    
    public class ApplicationDbContext : DbContext
    {
        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseSqlServer(@"Server=(localdb)\MSSQLLocalDB;Database=sample;Trusted_Connection=True");
        }

        public DbSet<FirstClass> FirstClasses { get; set; }
        public DbSet<SecondClass> SecondClasses {get; set; }
    }

This generates the table definition

CREATE TABLE [dbo].[SecondClasses] (
    [Id]           INT            IDENTITY (1, 1) NOT NULL,
    [FirstClassId] INT            NOT NULL,
    [Url]          NVARCHAR (MAX) NULL,
    CONSTRAINT [PK_SecondClasses] PRIMARY KEY CLUSTERED ([Id] ASC),
    CONSTRAINT [FK_SecondClasses_FirstClasses_FirstClassId] FOREIGN KEY ([FirstClassId]) REFERENCES [dbo].[FirstClasses] ([Id]) ON DELETE CASCADE
);


GO
CREATE UNIQUE NONCLUSTERED INDEX [IX_SecondClasses_FirstClassId]
    ON [dbo].[SecondClasses]([FirstClassId] ASC)

;