c# - Entity Framework 核心 : The property expression 'xx' is not valid. 表达式应表示属性访问: 't => t.MyProperty'

Question

尝试在 Entity Framework 核心的叶级别使用 where 子句进行查询，但出现此错误:

Unhandled Exception: System.ArgumentException: The property expression 'c => {from Child t in c.Children where ([t].Name != "NoInc
lude") select [t]}' is not valid. The expression should represent a property access: 't => t.MyProperty'. For more information on
including related data, see http://go.microsoft.com/fwlink/?LinkID=746393.
   at Microsoft.EntityFrameworkCore.Internal.ExpressionExtensions.GetComplexPropertyAccess(LambdaExpression propertyAccessExpressi
on)
   at Microsoft.EntityFrameworkCore.Query.ResultOperators.Internal.ThenIncludeExpressionNode.ApplyNodeSpecificSemantics(QueryModel
 queryModel, ClauseGenerationContext clauseGenerationContext)
   at Remotion.Linq.Parsing.Structure.IntermediateModel.MethodCallExpressionNodeBase.Apply(QueryModel queryModel, ClauseGeneration
Context clauseGenerationContext)
   at Remotion.Linq.Parsing.Structure.QueryParser.GetParsedQuery(Expression expressionTreeRoot)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileQueryCore[TResult](Expression query, INodeTypeProvider nod
eTypeProvider, IDatabase database, ILogger logger, Type contextType)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass19_0`1.<CompileQuery>b__0()
   at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQueryCore[TFunc](Object cacheKey, Func`1 compiler)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.Execute[TResult](Expression query)
   at Remotion.Linq.QueryableBase`1.GetEnumerator()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at dotnetsql.Demo.Main(String[] args) in C:\Users\asad\dev\dotnetsql\Program.cs:line 15

这里是完整的代码，可以在控制台应用程序中用来复制它:

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.EntityFrameworkCore;

namespace dotnetsql
{
    class Demo
    {
        static void Main(string[] args)
        {
            using (var context = new FamilyContext())
            {
                var seed = new SeedData(context);
                var result = context.GrandParent
                .Include(p => p.Parents)
                .ThenInclude(c => c.Children.Where(t => t.Name != "NoInclude"))
                .ToList();
                Console.WriteLine(result.Count);

            }
        }

    }

    public class FamilyContext : DbContext
    {
        public FamilyContext()
        {
            Database.EnsureCreated();
        }


        public DbSet<GrandParent> GrandParent { get; set; }
        public DbSet<Parent> Parent { get; set; }
        public DbSet<Child> Child { get; set; }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseInMemoryDatabase();
        }
    }
    public class GrandParent
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public List<Parent> Parents { get; set; }
    }
    public class Parent
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public int GrandParentId { get; set; }
        public GrandParent GrandParent { get; set; }
        public List<Child> Children { get; set; }
    }
    public class Child
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public int ParentId { get; set; }
        public Parent Parent { get; set; }
    }

    // Seed Data
    public class SeedData
    {
        public SeedData(FamilyContext context)
        {
            if (!context.GrandParent.Any())
            {
                context.GrandParent.AddRange(
                    new List<GrandParent>
                    {
                        new GrandParent{Name = "Grandparent 1", },
                        new GrandParent{Name = "Grandparent 2", },
                    }

                );
                context.SaveChanges();
                context.Parent.AddRange(
                    new List<Parent> {
                    new Parent{Name = "Parent 1", GrandParentId = 1},
                    new Parent{Name = "Parent 2", GrandParentId = 1},
                    }
                );
                context.SaveChanges();
                context.Child.AddRange(
                    new List<Child> {
                    new Child{Name = "Child 1", ParentId = 1},
                    new Child{Name = "Child 2", ParentId = 1},
                    new Child{Name = "NoInclude", ParentId = 1}
                    }
                );
                context.SaveChanges();
            }
        }


    }



}

阅读this文章，但延迟加载示例很简单，期望在过滤其子记录之前加载单个实体，这无济于事。

Here是用于快速复制此内容的完整控制台应用程序。

谢谢。

Answer 1

假设您想要执行一个过滤的包含 - 多次请求的功能并且在包括 Core 在内的任何 EF 版本中都不支持。据我所知，有计划将其添加到 EF Core，但在此之前您必须手动解决它。

该技术与 Loading Related Data 中解释的基本相同- 显式加载部分，过滤将哪些相关实体加载到内存中示例，但适用于任何查询。

您首先提取(但不执行)聚合根查询(在您的示例中为 GrandParents)并应用所有必要的过滤:

var query = context.GrandParent.AsQueryable();

然后执行并具体化主查询，应用非过滤包括:

var result = query
    .Include(p => p.Parents)
    .ToList();

最后显式加载预期的过滤包括:

query.SelectMany(p => p.Parents)
    .SelectMany(c => c.Children)
    .Where(t => t.Name != "NoInclude")
    .Load();