"The instance of entity type cannot be tracked" 添加分离的实体树时出错

Question

我无法添加交易实体，因为买方和卖方引用的是同一个城市“City1”。我在这里使用分离的实体，因为我通过 API.

接收整个模型

// This is the result of Deal entity deserialization
var deal = new Deal
{
    Id = "D1",
    Buyer = new Person
    {
        Id = "P1",
        Name = "Person1",
        City = new City { Name = "City1" }
    },
    Seller = new Person
    {
        Id = "P2",
        Name = "Person2",
        City = new City { Name = "City1" }
    }
};

_dbContext.Deals.Add(deal);
_dbContext.SaveChanges();

错误是

System.InvalidOperationException: 'The instance of entity type 'City' cannot be tracked because another instance with the same key value for {'Name'} is already being tracked. When attaching existing entities, ensure that only one entity instance with a given key value is attached. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see the conflicting key values.'

以下是实体的设置方式：

modelBuilder.Entity<City>(x =>
{
    x.HasKey(e => e.Name);
});

modelBuilder.Entity<Person>(x =>
{
    x.HasKey(e => e.Id);
    x.Property(e => e.Name);
    x.HasOne(e => e.City).WithMany();
});

modelBuilder.Entity<Deal>(x =>
{
    x.HasKey(e => e.Id);
    x.HasOne(e => e.Buyer).WithMany();
    x.HasOne(e => e.Seller).WithMany();
});

与此类实体合作的正确方法是什么？我使用 EF Core 6

Answer 1

您的两个 Person 对象需要为 City 引用同一个对象。或者，公开并使用 FK 属性（这似乎是城市的 Name 属性）而不是此操作的导航属性。

如果您想继续使用 JSON 中的 City 对象，则需要保留引用。例如，这里是 System.Text.Json

的文档

using System;
using System.Collections.Generic;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace PreserveReferences
{
    public class Employee
    {
        public string Name { get; set; }
        public Employee Manager { get; set; }
        public List<Employee> DirectReports { get; set; }
    }

    public class Program
    {
        public static void Main()
        {
            Employee tyler = new()
            {
                Name = "Tyler Stein"
            };

            Employee adrian = new()
            {
                Name = "Adrian King"
            };

            tyler.DirectReports = new List<Employee> { adrian };
            adrian.Manager = tyler;

            JsonSerializerOptions options = new()
            {
                ReferenceHandler = ReferenceHandler.Preserve,
                WriteIndented = true
            };

            string tylerJson = JsonSerializer.Serialize(tyler, options);
            Console.WriteLine($"Tyler serialized:\n{tylerJson}");

            Employee tylerDeserialized =
                JsonSerializer.Deserialize<Employee>(tylerJson, options);

            Console.WriteLine(
                "Tyler is manager of Tyler's first direct report: ");
            Console.WriteLine(
                tylerDeserialized.DirectReports[0].Manager == tylerDeserialized);
        }
    }
}

// Produces output like the following example:
//
//Tyler serialized:
//{
//  "$id": "1",
//  "Name": "Tyler Stein",
//  "Manager": null,
//  "DirectReports": {
//    "$id": "2",
//    "$values": [
//      {
//        "$id": "3",
//        "Name": "Adrian King",
//        "Manager": {
//          "$ref": "1"
//        },
//        "DirectReports": null
//      }
//    ]
//  }
//}
//Tyler is manager of Tyler's first direct report:
//True

Answer 2

如果稍微修改一下代码，就可以避免跟踪具有相同键的两个对象：

var newCity = new City { Name = "City1" };
_dbContext.Cities.Add(newCity);

var p1 = new Person
{
    Id = "P1",
    Name = "Person1",
    City = newCity
};
_dbContext.Persons.Add(p1);

var p2 = new Person
{
    Id = "P2",
    Name = "Person2",
    City = newCity
};
_dbContext.Persons.Add(p2);

var deal = new Deal
{
    Id     = "D1",
    Buyer  = p1,
    Seller = p2
};
_dbContext.Deals.Add(deal);

_dbContext.SaveChanges();

这样，您就告诉 Entity framework 这是同一个城市 - 您只是引用该对象并将该对象添加到城市列表中。保存将根据您的 table 定义以正确的顺序保留它。

同样，如果您仍然运行遇到问题，因为您添加了同一个人 - 首先创建人员对象并将其添加到人员列表中。

还有一件事 - 如果某个对象可能已经存在，请查询它。例如：

var cityName = "City3";
var city = _dbContext.Cities.FirstOrDefault(f => f.Name == cityName);
if (city == null)
{
   city = new City { Name = cityName };
   _dbContext.Cities.Add(city);
}
// from here on you can safely use city

"The instance of entity type cannot be tracked" 添加分离的实体树时出错

"The instance of entity type cannot be tracked" error when adding detached entity tree

c#

entity-framework

entity-framework-core