一对多连接 - 只取许多部分的最后一个

发布于 2024-09-25 22:20:10 字数 414 浏览 3 评论 0原文

我是 EF 的新手，所以如果我的问题之前已经得到解答，我很抱歉..我只是无法弄清楚语法..

我有两个实体，Category & 产品，其中一个类别有多种产品。

我想获取所有类别，仅包含他们的最新产品（它有一个名为时间戳的日期属性），

我不知道该怎么做。 :-/

如果可能的话我想知道两种编写方式的语法，类似 sql 语法和类似 C# 语法，例如：

ctx.Categories.Include("Products").ToList() 

from c in ctx.Categories.Include("Products")

谢谢！

原文

I'm quite a newbie in EF, so I'm sorry if my question has been answered before.. I just can't figure out the syntax..

I have two entities, Category & Product, where one category has many products.

I want to get all categories, with only their latest product (it has a date property named timestamp)

I have no idea how to do that. :-/

If possible I'd like to know the syntax of the two ways to write it, both the sql-like syntax, and the C# like syntax, e.g.:

ctx.Categories.Include("Products").ToList() 

from c in ctx.Categories.Include("Products")

Thanks!

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把昨日还给我 2024-10-02 22:20:10

这是类似于 SQL 的方式：

var categories =
        from p in products
        group p by p.Category into g
        select new { Category = g.TheKey, LatestProduct = g.Max(p => p.TimeStamp) };

这是 Lambda 方式（警告，未经测试）：

var categories = products.GroupBy(p => p.Category)
                         .Select(g => new { Category = g.TheKey, 
                                            LatestProduct = g.Max(p => p.TimeStamp)});

关于 Categories.Include("Products") 的注释，在您的示例中不需要它。您使用“包含”进行急切加载，因此，例如，如果您有从 EF 返回的类别列表，则当您执行 Categories.Product 时，您将获得关联的产品。

但您所需要的只是一个类别列表，以及每个类别的一个产品 - 该产品已在上面的 LINQ 查询中返回，因此不需要 Include。

Here's the SQL-like way:

var categories =
        from p in products
        group p by p.Category into g
        select new { Category = g.TheKey, LatestProduct = g.Max(p => p.TimeStamp) };

This is the Lambda-way (warning, untested):

var categories = products.GroupBy(p => p.Category)
                         .Select(g => new { Category = g.TheKey, 
                                            LatestProduct = g.Max(p => p.TimeStamp)});

A note on Categories.Include("Products"), you don't need this in your example. You use "Include" for eager-loading, so that for example if you had a list of Categories returned from EF, when you do Categories.Product you will get the associated product.

But all you require is a list of categories, and a single product for each one - which is already returned in the above LINQ query, so no need for Include.

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