加入/在何处使用 LINQ 和 Lambda [英] Join/Where with LINQ and Lambda

问题描述

我在使用 LINQ 和 Lambda 编写的查询时遇到问题.到目前为止，我收到了很多错误，这是我的代码:

I'm having trouble with a query written in LINQ and Lambda. So far, I'm getting a lot of errors here's my code:

int id = 1;
var query = database.Posts.Join(database.Post_Metas,
                                post => database.Posts.Where(x => x.ID == id),
                                meta => database.Post_Metas.Where(x => x.Post_ID == id),
                                (post, meta) => new { Post = post, Meta = meta });

我刚开始使用 LINQ，所以我不确定这个查询是否正确.

I'm new to using LINQ, so I'm not sure if this query is correct.

推荐答案

我发现如果您熟悉 SQL 语法，使用 LINQ 查询语法会更清晰、更自然，并且更容易发现错误:

I find that if you're familiar with SQL syntax, using the LINQ query syntax is much clearer, more natural, and makes it easier to spot errors:

var id = 1;
var query =
   from post in database.Posts
   join meta in database.Post_Metas on post.ID equals meta.Post_ID
   where post.ID == id
   select new { Post = post, Meta = meta };

不过，如果您真的坚持使用 lambda，那么您的语法就有点偏离了.这是使用 LINQ 扩展方法的相同查询:

If you're really stuck on using lambdas though, your syntax is quite a bit off. Here's the same query, using the LINQ extension methods:

var id = 1;
var query = database.Posts    // your starting point - table in the "from" statement
   .Join(database.Post_Metas, // the source table of the inner join
      post => post.ID,        // Select the primary key (the first part of the "on" clause in an sql "join" statement)
      meta => meta.Post_ID,   // Select the foreign key (the second part of the "on" clause)
      (post, meta) => new { Post = post, Meta = meta }) // selection
   .Where(postAndMeta => postAndMeta.Post.ID == id);    // where statement

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