INSERT 语句与 net core 中的 FOREIGN KEY 约束冲突

Question

I have 2 tables. one for Courses and another one for Keywords. These two table have many-to-many relations. So I put another table as CourseKeywords .

public class CourseKeyword
{
    public int CourseId { get; set; }
    public int KeywordId { get; set; }
    public virtual Course Course { get; set; }
    public virtual Keyword Keyword { get; set; }
}

public class Course
{
    [Key]
    public int Id { get; set; }
    [MaxLength(300)]
    public string Title { get; set; }
    [MaxLength(300)]
    public string SubTitle { get; set; }
    public string Body { get; set; }
   
    public ICollection<CourseKeyword> Keywords { get; set; }   

    public Guid Token { get; set; }
    [MaxLength(300)]
    public string Photo { get; set; }
    [MaxLength(300)]
    public string Preview { get; set; }
    public bool IsSpecial { get; set; }
}

public class Keyword
{
    [Key]
    public int Id { get; set; }
    [MaxLength(300)]
    public string Title { get; set; }
    public ICollection<CourseKeyword> CourseKeywords { get; set; }
    public ICollection<BlogKeyword> BlogKeywords { get; set; }
}

When I want to add a new course with some new keywords. I successfully add course. Then add keyword, but when I try to add CourseKeyword object, at SaveChange() it shows below error:

InnerException = {"The INSERT statement conflicted with the FOREIGN KEY constraint "FK_CourseKeywords_Course_KeywordId". The conflict occurred in database "PandaAcademyNew", table "dbo.Course", column 'Id'.\r\nThe statement has been terminated."}

This happen when I want to save change. My code is here:

var course = _db.Course.Find(model.Id);
var keyw = _db.Keywords.Find(keyword.Id);
    
var key = new CourseKeyword()
{
    Course = course,
    Keyword = keyw
};

_db.CourseKeywords.Add(key);
_db.SaveChanges();

This is my context OnModelCreating :

builder.Entity<CourseKeyword>(courseKeyword =>
{
    courseKeyword.HasKey(ur => new { ur.KeywordId, ur.CourseId });

    courseKeyword.HasOne(ur => ur.Course)
        .WithMany(r => r.Keywords)
        .HasForeignKey(ur => ur.CourseId)
        .IsRequired();

    courseKeyword.HasOne(ur => ur.Keyword)
        .WithMany(r => r.CourseKeywords)
        .HasForeignKey(ur => ur.KeywordId)
        .IsRequired();
});

Answer 1

You should try to create new CourseKeyword instance by keys not by entities. I mean:

var key = new CourseKeyword()
{
    CourseId = course.Id,
    KeywordId = keyw.Id
};

Answer 2

Try this code:

var key = new CourseKeyword()
{
    CourseId = course.Id,
    KeywordId = keyw.Id
};

_db.CourseKeywords.Add(key);
_db.SaveChanges();

and by the way fix your you dbContext:

builder.Entity<CourseKeyword>(courseKeyword =>
{
    courseKeyword.HasKey(ur => new { ur.KeywordId, ur.CourseId });

    courseKeyword.HasOne(ur => ur.Course)
        .WithMany(r => r.Keywords)
        .HasForeignKey(ur => ur.CourseId)
       .OnDelete(DeleteBehavior.ClientSetNull);

    courseKeyword.HasOne(ur => ur.Keyword)
        .WithMany(r => r.CourseKeywords)
        .HasForeignKey(ur => ur.KeywordId)
        .OnDelete(DeleteBehavior.ClientSetNull);

});