插入具有整数空值的行

Question

How to insert a Computer with an idUser as a new value?

public class User
{
    public User() 
    { 
    }
    [Key]
    public int idUser; { get; set; }
    [Required]
    public string UserName { get; set; }
}

public class Computer
{
    public Computer() 
    {
    }     
    [Key]   
    public int idComputer{ get; set; }
    public string ComputerName {get;set;}
    [ForeignKey("User")]
    public int? idUser{ get; set; }
    public virtual User User { get; set; }
}

I've tried this way:

PDbContext _db = new PDbContext();
Computer c = new Computer();
c.ComputerName = "sdsd";
c.User = null;
c.idUser = 0;
_db.Computers.Add(c);
_db.SaveChanges();

But this wouldn't work. Any brilliant suggestion please?

When I run that I get L'instruction INSERT est en conflit avec la contrainte FOREIGN KEY "FK_dbo.Poste_dbo.User_UserID".

Answer 1

Mark idUser as nullable.

public int? idUser { get; set; }

Additionally you could get rid of of the idUser property and just use the User navigation prop. Since User is marked virtual you can always access the UserId with user.Id.

This is how I would set up your Computer model:

public class Computer
{
    public int Id { get; set; }
    public string ComputerName { get;set; }
    public virtual User User { get; set; }
}

Or:

public class Computer
{
    public int Id { get; set; }
    public string ComputerName { get; set; }
    public int? UserId { get; set; }
    public User User { get; set; }
}

Using my first example you could insert like this:

PDbContext _db = new PDbContext();
Computer c = new Computer();
c.ComputerName = "sdsd";
c.User = null;
_db.Computers.Add(c);
_db.SaveChanges();

And with my second:

PDbContext _db = new PDbContext();
Computer c = new Computer();
c.ComputerName = "sdsd";
c.UserId = null;
_db.Computers.Add(c);
_db.SaveChanges();

Answer 2

[ForeignKey("User")]
public int? idUser{ get; set; }

This is part of your Computer class. Since not every Computer has a User, you've used a nullable-int so that you can use null to represent the case where there is no User.

However, then, you do this:

c.idUser = 0;

This isn't correct. You were using a nullable-int for a reason - you should be setting idUser to null here. Instead, you're using 0 as the value of your foreign key. Since there is no row in your users table with a UserID of 0, this fails with the error you're seeing.

Change to this, and you'll get a null in the UserID column of the new row; because it's a null, the foreign-key constraint won't apply:

c.idUser = null;