ASP.NET Core Conflict with Foreign Key

Question

I have got several models:

Course.cs

public class Course
{
    public Guid Id { get; set; }

    public ICollection<ApplicationUser> Teacher { get; set; }
    public string Name { get; set; }
    public string ShortName { get; set; }
    public DateTime CreationDate { get; set; }
    public bool IsActive { get; set; }
}

Group.cs

public class Group
{
    public Guid Id { get; set; }

    public ApplicationUser Mentor { get; set;}

    public string DisplayName { get; set; }
    public string GroupName { get; set; }
    public DateTime StartYear { get; set; }

    public string InviteCode { get; set; }

    public ICollection<ApplicationUser> Students { get; set; }
    public ICollection<Course> Courses { get; set; }
}

ApplicationUser.cs

public class ApplicationUser : IdentityUser
{
    [Required]
    public string Firstname { get; set; }
    [Required]
    public string Surname { get; set; }
    public bool Gender { get; set; }
    public DateTime Birthdate { get; set; }
    //[Required]
    public string InviteCode { get; set; }

    public Guid GroupId { get; set; }
    [ForeignKey("GroupId")]
    public Group CurrentGroup { get; set; }
    public ICollection<Group> PastGroups { get; set; }
}

Now when I try to register (using Identity) a user (not even trying to give the user a group) I receive this error:

SqlException: The INSERT statement conflicted with the FOREIGN KEY constraint "FK_AspNetUsers_Groups_GroupId". The conflict occurred in database "aspnet-Project_Dojo-3af15f80-8c62-40a6-9850-ee7a296d0726", table "dbo.Groups", column 'Id'. The statement has been terminated.

In my modelBuilder I have added some logics for the relations between Group, ApplicationUser (Students) and the Foreign Key:

protected override void OnModelCreating(ModelBuilder builder)
{
    base.OnModelCreating(builder);
    // Customize the ASP.NET Identity model and override the defaults if needed.
    // For example, you can rename the ASP.NET Identity table names and more.
    // Add your customizations after calling base.OnModelCreating(builder);\\

    builder.Entity<ApplicationUser>()
        .HasOne(p => p.CurrentGroup)
        .WithMany(b => b.Students)
        .HasForeignKey(p => p.GroupId);           
}

I don't know what this is exactly doing, but I've been browsing some Stackoverflow threads to come to this code (migrations weren't working without it).

I look forward to a solution for my problem. Once again, I'm not doing ANYTHING with the groups yet when registering.

Thanks in advance!

Answer 1

not even trying to give the user a group

Well there's your problem, it's required.

Either provide a group, or make it optional by making the foreign key nullable (Guid? GroupId).

Because it's currently a non-nullable struct, it'll have a default value of all zeroes (Guid.Empty). This FK is not known in your database, resulting in the error you see.