MVC C# Lambda Join 一对多选择一个答案

【问题标题】：MVC C# Lambda Join one to many select oneMVC C# Lambda Join 一对多选择一个
【发布时间】：2021-07-16 09:04:13
【问题描述】：

我需要加入 2 个表来列出类 DisplayProductList

public class DisplayProductList
{
    public Product_Details Product_Details { get; set; }
    public string Image_Path { get; set; }

    public DisplayProductList()
    {
        Product_Details = new Product_Details();
        Image_Path = string.Empty;
    }
}

表格

产品详情
产品图片

1 Product_Details 有许多 Product_Images 但我需要选择 Product_Images 的第一条记录加入

List<DisplayProductList> displayProductLists = new List<DisplayProductList>();
displayProductLists = db.Product_Details
    .Join(db.Product_Images.OrderBy(o => o.Product_Image_Seq),
    pd => pd.Product_Id,
    pi => pi.Product_Id,
    (pd, pi) => new { pd, pi })
    .Select(s => new DisplayProductList()
        {
            Product_Details = s.pd,
            Image_Path = s.pi.Product_Image_Path
        }).ToList();

这是我的命令。怎么改？

【问题讨论】：

标签： c# asp.net-mvc lambda

【解决方案1】：

有两种变体。

FirstOrDefault在投影中

var displayProductLists = db.Product_Details
    .Select(pd => new DisplayProductList()
    {
        Product_Details = pd,
        Image_Path = db.Product_Images
            .Where(pi => pi.Product_Id == pd.Product_Id)
            .OrderBy(pi => pi.Product_Image_Seq)
            .Select(pi => pi.Product_Image_Path)
            .FirstOrDefault()
    })
    .ToList();

更接近 SQL：

var query = 
    from pd in db.Product_Details
    from pi in db.Product_Images
        .Where(pi => pi.Product_Id == pd.Product_Id)
        .OrderBy(o => o.Product_Image_Seq)
        .Take(1)
        .DefaultIfEmpty()
    select new DisplayProductList
    {
        Product_Details = pd,
        Image_Path = pi.Product_Image_Path
    };

var displayProductLists = query.ToList();

【讨论】：