无法在连接表实体框架中两次保存相同的记录

Question

I have 2 tables, Invoice and Product , and I'm using EF to save data to my database

The two classes and my context class are as following:

public class Invoice
{
        public int Id { get; set; }
        public string InvoiceNumber { get; set; }
        public double TotalPrice { get; set; }
        public List<Product> Products { get; set; }
}
public class Product
{
        public int Id { get; set; }
        public string Name { get; set; }
        public double Price { get; set; }
        public List<Invoice> Invoices { get; set; }
}
public class DB : DbContext
{
    public DB() : base("MyConnectionstringName") { }

    public DbSet<Invoice> Invoices { get; set; }
    public DbSet<Product> Products { get; set; }
}

Finally, this is how I save data to the database:

public string Create(Invoice invoice, List<Product> products)
{
        foreach (var item in products)
        {
            invoice.Products.Add
                (db.Products.Find(item.id));
        }
        db.Invoices.Add(invoice);
        db.SaveChanges();
        ...
        return "Invoice was successfully saved";
}

The problem is, if I have the same Product twice in the products , I gets added to the join table in the database, only once. For example, if a customer buys two Pepperoni pizza which has the Id of 5, after saving, there will be only of row of data in InvoiceProduct pointing to this particular invoice, and ProductId of 5.

Everything is fine up until db.Invoices.Add(invoice); , cause when I check invoice in debug mode I can see several of the same item were added to its Products property, but when it gets saved to the database, it only get saved once.

And it works just fine with several of different products for the same invoice.

What am I not seeing here?

Answer 1

In sql relational world you have the Products table, the Invoice table and the ProductsInvoices table that represents a many-to-many relationship. In that table you have the reference for an id of a product and an id of an invoice. When you add it twice EF understand that you already have the relation you want and does nothing.

In order to solve this I would create other class named InvoiceItem having a Product, an Invoice and a Quantity:

public class Invoice
{
        public int Id { get; set; }
        public string InvoiceNumber { get; set; }
        public double TotalPrice { get; set; }
        public List<InvoiceItem> Items { get; set; }
}

public class InvoiceItem
{
        public int Id { get; set; }
        public int Quantity { get; set; }
        public Product Product { get; set; }
}

public class Product
{
        public int Id { get; set; }
        public string Name { get; set; }
        public double Price { get; set; }
        public List<InvoiceItem> Invoices { get; set; }
}

public class DB : DbContext
{
    public DB() : base("MyConnectionstringName") { }

    public DbSet<Invoice> Invoices { get; set; }
    public DbSet<InvoiceItem> InvoiceItems { get; set; }
    public DbSet<Product> Products { get; set; }
}

It is recommended that you also add the copy of the item description and price of the Product in the InvoiceItem object, because if in the future the user changes the price or name of the product it doesn't alter the Invoice that were generated in the past.