为什么我从数据库中获取空对象？

Question

我有一个具有属性的发票对象，其中有一个客户端对象和一个项目列表。我的目标是从数据库中检索发票列表，但每次我这样做时，发票都会返回分配给其客户端对象和项目列表的空引用。我的数据模型有问题吗？

这是我的数据模型：

public class Invoice
    {
        public string InvoiceId { get; set; } = GenerateID.GenerateInvoiceID();
        public string Description { get; set; }
        public List<InvoiceItem> Items { get; set; }
        public DateTime InvoiceDate { get; set; }
        public string PaymentTerms { get; set; }
        public DateTime PaymentDue { get; set; }
        public int TotalPrice { get; set; }
        public string Status { get; set; } = "pending";
        public Client Client { get; set; }
 

        public string BillFromAddress { get; set; }
        public string BillFromCity { get; set; }
        public string BillFromCountry { get; set; }
        public string BillFromPostal { get; set; }

    }



public class Client
    {
        public string ClientId{ get; set; } = GenerateID.GenerateClientID();
        
        [Required]
        public string ClientName { get; set; }
        [Required]
        public string ClientEmail { get; set; }
        [Required]
        public string ClientAddress { get; set; }
        [Required]
        public string ClientCity { get; set; }
        [Required]
        public string ClientCountry { get; set; }
        [Required]
        public string ClientPostal { get; set; }

    }

public class InvoiceItem
    {
        public string InvoiceItemId { get; set; } = GenerateID.GenerateItemID();
        public string InvoiceId { get; set; }
        [Required]
        public string Name { get; set; }
        [Required]
        public int Quantity { get; set; }
        [Required]
        public int Price { get; set; }

        public InvoiceItem()
        {
        }

        public InvoiceItem(string itemName, int quantity, int price)
        {
            Name = itemName;
            Quantity = quantity;
            Price = price;
        }

    }

这是添加和检索发票的服务：

public async void AddInvoice(InputModel input)
        {
            Invoice invoice = new Invoice();
           
            invoice.Description = input.Description;
            invoice.Items = input.Items;
            invoice.InvoiceDate = input.InvoiceDate;
            invoice.PaymentTerms = input.PaymentTerms;
            invoice.Client = input.Client;

            
            invoice.BillFromAddress = input.BillFromAddress;
            invoice.BillFromCity = input.BillFromCity;
            invoice.BillFromCountry = input.BillFromCountry;
            invoice.BillFromPostal = input.BillFromPostal;

            //Attaching the invoice id to each item in the invoice
            foreach (var item in invoice.Items)
            {
                item.InvoiceId = invoice.InvoiceId;
            }

            
            _context.Add(invoice);
            await _context.SaveChangesAsync();
        }

public async Task<List<Invoice>> GetInvoices()
        {
            return await _context.Invoices.ToListAsync();
        }

这是 DbContext 类：

public class AppDbContext : DbContext
    {
        public AppDbContext(DbContextOptions<AppDbContext> options) : base(options)
        {
        }

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

这是数据库中的记录：

Answer 1

为了能够查询Invoice的Client、InvoiceItem等相关实体，您需要预先加载。

public async Task<List<Invoice>> GetInvoices()
{
    return await _context.Invoices
       .Include(x => x.Client) 
       .Include(x => x.Items) 
       .ToListAsync();
}

参考资料

预加载