EF Core insert new entity with existing children

Question

I'm unsure how to use EF Core to insert a new entity that has existing children.

I have a list of Products in the database. I am attempting to create a new order that has a list of existing products.

This is the Product class:

public class Product
{
    public long ID { get; set; }

    public string Name { get; set; }
    public string Description { get; set; }

    [Column(TypeName = "decimal(8, 2)")]
    public decimal Price { get; set; }

    public string Category { get; set; }
}

And this is the Order class

public class Order
{
    public long ID { get; set; }
    public string Name { get; set; }
    public string City { get; set; }
    public string State { get; set; }
    public string Zip { get; set; }
    public string Country { get; set; }
    public bool Giftwrap { get; set; }

    public IEnumerable<Product> Products { get; set; }
}

When I attempt to insert a new order with existing products:

[HttpPost]
public IActionResult Post([FromBody]  Order model)
{
    context.Add(model);
    context.SaveChanges();
    return Ok(model);
}

I get an error:

SqlException: Cannot insert explicit value for identity column in table 'Products' when IDENTITY_INSERT is set to OFF.

This is the DbContext:

using Microsoft.EntityFrameworkCore;

namespace angularsportsstorenetcore2.Models
{
    public class StoreDbContext : DbContext
    {
        public StoreDbContext(DbContextOptions<StoreDbContext> options)
            : base(options) { }

        public DbSet<Product> Products { get; set; }
        public DbSet<Order> Orders { get; set; }
    }
}

Does the incoming serialized Product array contains valid IDs? If any of them is 0, this flow will try to insert the new product with id as 0 — MKougiouris, Oct 04 '21 at 13:43
Instead of `Add` use `Update`, it does what in the non-core version of Entity Framework `AddOrUpdate` does. [This](https://stackoverflow.com/q/62449078/9363973) Q&A might help you solve your problem — MindSwipe, Oct 04 '21 at 13:45
@MKougioris yes the serialized product array does contain the valid ids of the I will update my question. — Bryan Dellinger, Oct 04 '21 at 13:46
This doesn't make sens ... there should be 3 Tables Orders, OrderProducts(OrderItems) and Products ... if saving `Order` trying to save to `Products` then you does some wrong configuration of many to many relationship — Selvin, Oct 04 '21 at 13:48
New parent cannot have existing children. Most probably the relationship is incorrect - Product is not child of Order (neither Order is child of Product), hence you seem to need many-to-many relationship. — Ivan Stoev, Oct 04 '21 at 13:50

score 0 · Answer 1 · answered Oct 04 '21 at 14:57

Thank you all took @Selvin's advice and created a many to many relationship.

   public class OrderProductJunction
    {
        public long Id { get; set; }
        public long OrderId { get; set; }
        public Order Order { get; set; }
        public long ProductId { get; set; }
        public Product Product { get; set; }
    } 



 public class Order
    {
        public long ID { get; set; }
        public string Name { get; set; }
        public string City { get; set; }
        public string State { get; set; }
        public string Zip { get; set; }
        public string Country { get; set; }
        public bool Giftwrap { get; set; }
        [NotMapped]
        public IEnumerable<Product> Products { get; set; }
        public IEnumerable<OrderProductJunction> OrderProductJunctions { get; set; }

    }

public class Product { public long ID { get; set; }

    public string Name { get; set; }

    public string Description { get; set; }

    [Column(TypeName = "decimal(8, 2)")]
    public decimal Price { get; set; }

    public string Category { get; set; }

    public IEnumerable<OrderProductJunction> OrderProductJunctions { get; set; }
}

and I changed the controller to.

 public IActionResult Post([FromBody]  Order model)
        {
            context.Add(model);
            context.SaveChanges();
            foreach (var item in model.Products)
            {
                context.Add(new OrderProductJunction { OrderId = model.ID, ProductId = item.ID });
            }
            context.SaveChanges();
            return Ok(model);
        }

don't call 2 times SaveChanges(), you can call only last time. — Den, Oct 04 '21 at 15:11
@Den, then how does EF know what the OrderProductJunction.OrderId is? — johnw182, Nov 07 '22 at 15:23
@johnw182 you can follow this [link](https://learn.microsoft.com/en-us/ef/core/saving/related-data) — Den, Nov 09 '22 at 18:44

EF Core insert new entity with existing children

1 Answers1