c# - Save data with many to many relationship EF core

Question

I have a cart which can have many products and a product can be in many carts I want to add product to the cart but I'm confused how I should do it? Is saving in the CartProduct table right or something else if someone can explain to me how this works because I'm lost.

Cart class:

public class Cart
{
    public int Id { get; set; }
    public int ProductCount { get; set; }
    public AppUser User { get; set; }

    public List<CartProduct> CartProduct { get; set; }
}

Product class:

public partial class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
    public decimal Price { get; set; }
    public string Category { get; set; }
    public decimal CostToMake { get; set; }
    public decimal FinalPrice { get; set; }
    public int? OrderId { get; set; }
    public int? CartId { get; set; }

    public virtual Order Order { get; set; }
    public List<CartProduct> CartProduct { get; set; }
}

CartProduct class:

public class CartProduct
{
     public int CartId { get; set; }
     public int ProductId { get; set; }
  
     public Cart Cart { get; set; }
     public Product Product { get; set; }
}

Car service:

public class SqlCart : ISqlCart
{
    private ManagerZContext _dbContext;

    public SqlCart(ManagerZContext dbContext)
    {
        _dbContext = dbContext;
    }

    public Cart AddProductToCart(Product product, Cart cart)
    {
        List<CartProduct> cartList = _dbContext.CartProduct.Where(cp => cp.CartId == cart.Id).ToList(); 
        cartList.Add(product);

        cart.CartProduct = 
        cart.ProductCount = cartList.Count();

        _dbContext.Attach(cart).State = EntityState.Modified;
        _dbContext.SaveChanges();

        return new Cart();
    }

    public Cart CreateCart(AppUser user)
    {
        Cart cart = new Cart();
        cart.User = user;
        _dbContext.Carts.Add(cart);
        return new Cart();
    }

    public Cart GetCart(AppUser user)
    {
        Cart cart = _dbContext.Carts.Where(c => c.User == user).FirstOrDefault();
        List<CartProduct> cartList = _dbContext.CartProduct.Where(cp => cp.CartId == cart.Id).ToList();
        cart.CartProduct = cartList;

        return cart;
    }
}

I'm confused what to do in the Cart service to add products to the cart?

question from:https://stackoverflow.com/questions/65870454/save-data-with-many-to-many-relationship-ef-core

Answer 1

Modify your AddProductToCart method like -

public Cart AddProductToCart(Product product, Cart cart)
{
    var newCartProduct = new CartProduct { Cart = cart, Product = product };

    // OR
    // var newCartProduct = new CartProduct { CartId = cart.Id, ProductId = product.Id };

    _dbContext.CartProduct.Add(newCartProduct);
    _dbContext.SaveChanges();

    return cart;
}

You must have Ids with the product and cart parameters.