LINQ：多表/连接+分组
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【中文标题】LINQ：多表/连接+分组【英文标题】：LINQ: Multi table/ join + Group by 【发布时间】：2021-02-25 23:22:12 【问题描述】：我有一组非常不寻常的表，我需要为其编写一个 Linq 查询并返回一个特定的自定义模型，以下是实体及其关系
public class CustomerAccount
public Guid Id get; set;
public virtual Shop Shop get; set;
public Guid ShopId get; set;
public virtual Account Account get; set;
public Guid AccountId get; set;
public string MyAreaName get; set;
public virtual ICollection<CustomerAccountTag> CustomerAccountTags get; set;
public class Shop
public Guid Id get; set;
public string Name get; set;
public class Account
public Guid Id get; set;
public string Name get; set;
public class CustomerAccountTag
public Guid CustomerAccountId get; set;
public string TagId get; set;
public virtual CustomerAccount CustomerAccount get; set;
public virtual Tag Tag get; set;
public class Tag
public Guid Id get; set;
public string Name get; set;
public virtual ICollection<CustomerAccountTag> CustomerAccountTags get; set;
CustomerAccount 包含客户列表以及他们的商店和帐户详细信息，以及一些元数据。
它将包含如下记录
"CustomerAccount": [
"Id": "1",
"ShopId": "Shop1",
"AccountId": "AccountA"
,
"Id": "2",
"ShopId": "Shop1",
"AccountId": "AccountB"
,
"Id": "3",
"ShopId": "Shop2",
"AccountId": "AccountC"
]
CustomerAccountTag 表只是 CustomerAccount 和 Tags 表之间的链接表，标签是分配给每个 CustomerAccount 行的字符串。
我需要以下输出， 按商店 ID 分组（因此上面的示例将有两行数据，一行用于 shop1，另一行用于 shop2） 每行将有以下作为数据
ShopName
ShopId
Accounts = "custom list of accounts per shop, with Account Id, Account Name, AreaName and a list of related Tags"
以下是我如何完成它并且它可以工作，但仅适用于 LINQ lambda，问题是我不能“分组”，除非我将连接的表作为列表返回，然后我可以处理它。否则它会抱怨它无法在服务器上运行查询，需要在客户端上进行评估！
var queryResults = (from e in _applicationDbContext.CustomerAccount
.Include(g => g.Shop)
.Include(y => y.Account)
.Include(a => a.CustomerAccountTag)
.ThenInclude(t => t.Tag)
select e).ToList();
var result = queryResults.GroupBy(x => x.Shop)
.Select(y => new CustomAccountResponse
ShopId = y.Key.Id,
ShopName = y.Key.Name,
Accounts = y.Select(x => new y.Account.Name, y.Account.Id, y.AreaName, string.Join(",", y.CustomerAccountTags.Select(x => x.Tag.Name)) )
);
想知道我是否可以用 Linq 做同样的事情，但在数据库上运行所有内容，而不是一半在客户端，一半在客户端！
更新：
我认为下面的示例代码可以解释这个问题，也许 EFCore 不支持我的要求
using Microsoft.EntityFrameworkCore;
using System;
using System.Collections.Generic;
using System.Linq;
namespace ConsoleApp2
public static class Program
public static void Main()
using (var context = new ApplicationDbContext())
context.Database.EnsureDeleted();
context.Database.EnsureCreated();
var shop1 = new Shop Name = "Shop1" ;
var shop2 = new Shop Name = "Shop2" ;
var shop3 = new Shop Name = "Shop3" ;
var shop4 = new Shop Name = "Shop4" ;
var shop5 = new Shop Name = "Shop3" ;
List<Shop> shop = new List<Shop>() shop1, shop2, shop3, shop4, shop5 ;
context.Shops.AddRange(shop);
context.SaveChanges();
using (var context = new ApplicationDbContext())
var query = from shop in context.Shops
group shop by shop.Name into grouped
select new
Id = grouped.Key,
Count = grouped.Count(),
Items = grouped
;
/*
Processing of the LINQ expression 'GroupByShaperExpression:
KeySelector: s.Name,
ElementSelector:EntityShaperExpression:
EntityType: Shop
ValueBufferExpression:
ProjectionBindingExpression: EmptyProjectionMember
IsNullable: False
' by 'RelationalProjectionBindingExpressionVisitor' failed. This may indicate either a bug or a limitation in Entity Framework. See https://go.microsoft.com/fwlink/?linkid=2101433 for more detailed information.
*/
Console.ReadLine();
public class Shop
public Guid Id get; set;
public string Name get; set;
public class ApplicationDbContext : DbContext
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
base.OnConfiguring(optionsBuilder);
optionsBuilder.UseSqlServer("Server=(localdb)\\mssqllocaldb;Database=testdb;Trusted_Connection=True;MultipleActiveResultSets=true");
protected override void OnModelCreating(ModelBuilder modelBuilder)
base.OnModelCreating(modelBuilder);
public DbSet<Shop> Shops get; set;
我真的可以在我的查询中获得分组项目的列表吗？ 这正是发生以下异常的地方
/*
Processing of the LINQ expression 'GroupByShaperExpression:
KeySelector: s.Name,
ElementSelector:EntityShaperExpression:
EntityType: Shop
ValueBufferExpression:
ProjectionBindingExpression: EmptyProjectionMember
IsNullable: False
' by 'RelationalProjectionBindingExpressionVisitor' failed. This may indicate either a bug or a limitation in Entity Framework. See https://go.microsoft.com/fwlink/?linkid=2101433 for more detailed information.
*/
【问题讨论】：
【参考方案1】：您已创建正确的查询。没有简单的方法可以使用 EF 在服务器上完全运行此查询。您可以在此处执行的操作 - 限制从服务器检索的数据并强制 EF 生成最佳查询。
var query =
from a in _applicationDbContext.CustomerAccount
from t in a.CustomerAccountTags
select new
Shop = new a.Shop.Id, a.Shop.Name ,
Account = new a.Account.Name, a.Account.Id, a.AreaName ,
TagName = t.Tag.Name
;
var result =
from q in query.AsEnumerable()
group q by q.Shop into g
select new CustomAccountResponse
ShopId = g.Key.Id,
ShopName = g.Key.Name,
Accounts = g.GroupBy(x => x.Account).Select(x => new
x.Key.Account.Name,
x.Key.Account.Id,
x.Key.Account.AreaName,
Tags = string.Join(",", x.Select(y => y.TagName))
).ToList()
;
【讨论】：
非常感谢，是的，您的方法有效。也许我对 EFCore 支持的内容感到困惑，我已经用示例代码更新了我的帖子，请您看看是否能解释这个问题？ 你不能那样做。项目无法选择***.com/questions/64377955/…以上是关于LINQ：多表/连接+分组的主要内容，如果未能解决你的问题，请参考以下文章