ASP.NET MVC - Cannot insert duplicate key row in object

Question

I have a problem with Entity Framework Core. I have 2 models: Category.cs

    public int Id { get; set; }
    [DisplayName("Category | ")]
    [Required(ErrorMessage = "Category Name is Required")]
    public string CategoryName { get; set; }

And LibraryItem.cs

    public int Id { get; set; }

    [DisplayName("Category Id")]
    [Required(ErrorMessage = "Category Id is Required")]
    public int CategoryId { get; set; }

    public Category Category { get; set; }
    [Required(ErrorMessage = "Title is Required")]
    public string Title { get; set; }

    [DisplayName("Author, Speaker or Director")]
    [Required(ErrorMessage = "Creator of the media is required")]
    public string Author { get; set; }

    public int? Pages { get; set; }

    [DisplayName("DVD or Audiobook Length")]
    public int? RunTimeMinutes { get; set; }

    [DisplayName("Available to borrow")]
    public bool IsBorrowable { get; set; }

    [DisplayName("Name of Borrower")]
    public string Borrower { get; set; }

    [DisplayName("Date of the Borrow")]
    public DateTime? date { get; set; }

    [DisplayName("Type of Media")]
    [Required(ErrorMessage = "Type of Media is Required")]
    public string Type { get; set; }

LibraryItem has a Foreign key of CategoryId, which is the Primary key for the Category table. I wanna add two items with the same foreign key. So the two items are within the same Category. But when I try to add another one with the same CategoryId I get this error

cannot insert duplicate key row in object

Does anyone know what might be causing this problem? Many thanks!

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– user47589, Commented Apr 3, 2021 at 15:09

Serge · Accepted Answer · 2021-04-03 14:58:56Z

2

The problem is that you created one-to-one relation, but you need one-to-many

You have to fix you Category class:

    public int Id { get; set; }
    [DisplayName("Category | ")]
    [Required(ErrorMessage = "Category Name is Required")]
    public string CategoryName { get; set; }

    [InverseProperty(nameof(Library.Category))]
    public virtual ICollection<LibraryItem> LibraryItems {get; set:}

After fixing you will have to make a migration again

edited Apr 3, 2021 at 14:58

answered Apr 3, 2021 at 14:53

Serge

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Comments

Besnik · Accepted Answer · 2021-04-03 15:27:38Z

1

You have to let EF know that these two entities are related in a one to many relationship by adding the navigation properties on each class:

    public class Category {
        ...
        
        public virtual ICollection<LibraryItem> LibraryItems {get; set;}
    }


    public class LibraryItem {
        ...
        
        [DisplayName("Category Id")]
        [Required(ErrorMessage = "Category Id is Required")]
        public int CategoryId { get; set; }

        public Category Category { get; set; }
    }

If you add the navigation properties, you can use the Fluent API to manually configure relationships inside your DbContext's OnModelCreating method

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<LibraryItem>()
            .HasOne(l => l.Category)
            .WithMany(c => c.LibraryItems);
    }

answered Apr 3, 2021 at 15:27

Besnik

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1 Comment

Joro Over a year ago

Hi! Thanks for explaining it to me, it makes sense now. :)

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