我正在将 EF6.x 项目更新到 EF Core 3.1。决定回到基础，并以如何再次从头开始建立关系为例。
根据微软官方文档EF Core Relationship Examples，我将示例翻译成控制台应用程序如下：
using Microsoft.EntityFrameworkCore;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
namespace BlogPostsExample
{
class Program
{
async static Task Main(string[] args)
{
// SQL Running in a Docker container - update as required
var conString = "data source=localhost,14330;initial catalog=BlogsDb;persist security info=True;user id=sa;password=<Your super secure SA password>;MultipleActiveResultSets=True;App=EntityFramework;";
var ctx = new MyContext(conString);
await ctx.Database.EnsureCreatedAsync();
var result = await ctx.Posts.GroupBy(p => p.Blog).ToArrayAsync();
}
}
class MyContext : DbContext
{
private readonly string _connectionString;
public MyContext(string connectionString)
{
_connectionString = connectionString;
}
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
base.OnConfiguring(optionsBuilder);
if (!optionsBuilder.IsConfigured)
{
optionsBuilder
.UseSqlServer(_connectionString);
}
}
public DbSet<Blog> Blogs { get; set; }
public DbSet<Post> Posts { get; set; }
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<Post>()
.HasOne(p => p.Blog)
.WithMany(b => b.Posts)
.HasForeignKey(p => p.BlogId) //Tried with and without these keys defined.
.HasPrincipalKey(b => b.BlogId);
}
}
public class Blog
{
public int BlogId { get; set; }
public string Url { get; set; }
public List<Post> Posts { get; set; }
}
public class Post
{
public int PostId { get; set; }
public string Title { get; set; }
public string Content { get; set; }
public int BlogId { get; set; }
public Blog Blog { get; set; }
}
}
数据库中没有数据。EF Core 无法转换
ctx.Posts.GroupBy(p => p.Blog)
到商店查询。在我看来，这似乎是您可以尝试的最简单的 GroupBy 示例。
当您运行此代码时，您会收到以下异常：
System.InvalidOperationException: 'The LINQ expression 'DbSet<Post>
.Join(
outer: DbSet<Blog>,
inner: p => EF.Property<Nullable<int>>(p, "BlogId"),
outerKeySelector: b => EF.Property<Nullable<int>>(b, "BlogId"),
innerKeySelector: (o, i) => new TransparentIdentifier<Post, Blog>(
Outer = o,
Inner = i
))
.GroupBy(
source: p => p.Inner,
keySelector: p => p.Outer)' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to either AsEnumerable(), AsAsyncEnumerable(), ToList(), or ToListAsync(). See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.'
让它工作的唯一方法是在GroupBy.
从性能的角度来看，这显然不是很好，它将分组操作变成了客户端操作，您确实希望在服务器端进行分组。
我错过了什么明显的东西吗？我很难相信 EF Core 不能按照 EF 框架从第一天起就一直在做的最简单的组。这似乎是任何数据驱动应用程序的基本要求？（或任何具有少量数据的应用程序！）
更新：
添加属性，例如相关博客的主键没有任何区别。
更新 2：
如果您关注这篇 JetBrains 文章，您可以这样做：
var ctx = new EntertainmentDbContext(conString);
await ctx.Database.EnsureCreatedAsync();
var dataTask = ctx
.Ratings
.GroupBy(x => x.Source)
.Select(x => new {Source = x.Key, Count = x.Count()})
.OrderByDescending(x => x.Count)
.ToListAsync();
var data = await dataTask;
但不是这个：
var ctx = new EntertainmentDbContext(conString);
await ctx.Database.EnsureCreatedAsync();
var dataTask = ctx
.Ratings
.GroupBy(x => x.Source)
// .Select(x => new {Source = x.Key, Count = x.Count()})
// .OrderByDescending(x => x.Count)
.ToListAsync();
var data = await dataTask;
它仅适用于聚合函数，例如上面的计数。
SQL中类似的东西有效
SELECT COUNT(R.Id), R.Source
FROM
[EntertainmentDb].[dbo].[Ratings] R
GROUP BY R.Source
但是，删除聚合函数，COUNT 不会，您会收到类似于以下内容的消息：
Column 'EntertainmentDb.dbo.Ratings.Id' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.
所以看起来我想问 EF Core 一个我不能在 TSQL 中问的问题