带有GroupBy的LINQ求和

（不确定是否需要GroupBy）

我的（简化）表格：

Products（产品ID、名称、代码）
Invoices（InvoiceID，Number，IsPayed）
Invoices_Products（InvoiceID，ProductID，Quantity，Price）-多对多链接表

我需要显示一个按产品代码分组的已付款发票的发票_产品列表，该产品代码总计（数量*价格）。

我第一次用来获得可以绑定到UI的集合的代码：

IEnumerable<Invoices_Products> invoices_products = db.Invoices_Products
.Where(ip => ip.Invoice.IsPaid).DistinctBy(m => m.Product.Code);

然后，我对其进行迭代，将其绑定到UI:

List<BindableInvoiceProduct> bindableInvoiceProducts = 
new List<BindableInvoiceProduct>();
foreach (var item in invoices_products)
{
    decimal salesValue = db.Invoices_Products.Where(ip => ip.Invoice.IsPaid 
    && ip.Product.Code == item.Product.Code).Sum(m => (m.Price * m.Quantity));
    bindableInvoiceProducts.Add(new BindableInvoiceProduct()
    {
        A = item.A,
        B = item.B,
        SalesValue = salesValue.ToString()
    });
}

（DistinctBy方法来自morelinq）

为什么总的来说不正确？

编辑：

一些数据：

产品-产品ID=1，名称=123，代码=A
产品-产品ID=2，名称=456，代码=A
发票-InvoiceID=1，Number=INV123，IsPayed=True
Invoices_Products-InvoiceID=1，ProductID=1，数量=10，价格=100
Invoices_Products-InvoiceID=1，ProductID=2，数量=10，价格=200

预期结果：

代码=A，销售价值=3000

from invoice in invoices
where invoice.IsPaid
from xr in invoice.InvoiceProducts
group xr.Quantity * xr.Price by xr.Product.Code into g
select new {Code = g.Key, SalesValue = g.Sum()};

from invoice in invoices
where invoice.IsPaid
from xr in invoice.InvoiceProducts
group xr.Quantity * xr.Price
  by new {Code = xr.Product.Code, Invoice = invoice }
  into g
select new {
  Code = g.Key.Code,
  Invoice = g.Key.Invoice,
  SalesValue = g.Sum()};

var bindableInvoiceProducts = db.Invoices_Products
    .Where(ip => ip.Invoice.IsPaid)
    .GroupBy(ip => ip.Product.Code, 
             (code, ips) => new BindableInvoiceProduct()
                {
                    Code = code,
                    SalesValue = ips.Sum(ip => (ip.Price*ip.Quantity))
                })
    .ToList();