实体框架一对一关系错误

Question

I am trying to save a sale in the database, the sale has a user, when I saved the first sale with the code that I attached I had no problem, but from there, every time I went to save another resulting sale in the following Error:

Microsoft.EntityFrameworkCore.DbUpdateException: 'An error occurred while updating the entries. See the inner exception for details.'

SqlException: Cannot insert duplicate key row in object 'dbo.sale' with unique index 'IX_sale_userId'. The duplicate key value is (2). The statement has been terminated.

What am I doing wrong?

Sale Form: private void button2_Click(object sender, EventArgs e) {

            using (var context = new AppDbContext())
            {
                sale V = new sale();
                V.date = DateTime.Today.Date;
                V.userId = 2;
                context.Add(V);
                context.SaveChanges();


            }

}

Sale:

    public class sale
{
    [Key]
    public int saleId { get; set; }
    public DateTime date { get; set; }

   public int userId { get; set; }

    public virtual user user { get; set; }

}

User:

public class user
    {
        [Key]
        public int useroId { get; set; }
        public string nape { get; set; }
        public string surname { get; set; }
        public string email { get; set; }
        public string password { get; set; }
       public virtual sale sale { get; set; }

    }

AppDBContext:

 protected override void OnModelCreating(ModelBuilder modelBuilder)
    {



    modelBuilder.Entity<sale>().HasOne(a => a.user);
        base.OnModelCreating(modelBuilder);

    }

Answer 1

Try the following fluent Mapping

protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
    modelBuilder.Entity<sale>()
    .HasOne(a => a.user)
   .WithOne(b => b.sale)
   .HasForeignKey<user>(b => b.userId);
   }