实体框架中按属性求和的相关集合的总和（延迟加载）

Question

我在实体框架查询方面需要一些帮助。 我有以下实体：

public class Invoice
{
    public Guid Id { get; set; }
    public DateTime CreateDate { get; set; }
    [ForeignKey("CreatedById")]
    public virtual ApplicationUser CreatedBy { get; set; }
    public Guid CreatedById { get; set; }
    public bool Approved { get; set; }
    public virtual ICollection<InvoiceDetail> Details { get; set; }
}

public class InvoiceDetail
{
    public Guid Id { get; set; }
    [ForeignKey("InvoiceId")]
    public virtual Invoice Invoice { get; set; }
    public Guid InvoiceId { get; set; }
    public string Item { get; set; }
    public string Description { get; set; }
    public decimal Quantity { get; set; }
    public decimal UnitCost { get; set; }
    public decimal Discount { get; set; }
}

我正在尝试汇总批准发票的所有项目的总和。 我遇到的问题是，一旦我为“已批准” = 1进行过滤，便会到达InvoiceDetail级别的语法：

var myInvoices = context.Invoices.Where(i => i.CreatedById == userId).Include(i => i.CreatedBy).Include(i => i.Details);

var approvedTotal = myInvoices.Where(i => i.Approved == 1).Select([GET TO DETAILS???]);

// my goal is to get the following sum for each detail for all of the approved invoices:
// ((d.Quantity * d.UnitCost) - d.Discount)

Answer 1

假设您希望在执行给定计算后获得所有详细信息的总和，则可以执行以下操作：

var approvedTotal = invoices.Where(invoice => invoice.Approved)
            .Select(invoice => invoice.Details.Sum(detail => ((detail.Quantity * detail.UnitCost) - detail.Discount)));

因此，这给您每张发票一个总额。