当我们使用类的新实例时，EF将始终尝试将其插入DB。解决此问题的一种方法是“撤消” Changetracker内部存储的内容，因此它仅使用从DB加载的实体。

所以我所做的就是覆盖 SaveChanges() 方法，循环遍历每个 Department 里面 ChangeTracker，获取一份教师清单 Ids 每个，清除 ChangeTracker 所有人的条目 Faculty 对象，然后再次将它们添加到他们的部门，但现在使用在任何一个中找到的实体 ChangeTracker 或通过 Find().

它看起来有点低效，所以我进行了一个测试，一个使用此方法，另一种加载 Faculty 从每次运行的DB。我跑了10,000次，结果平均为三次：

Enum list:    77642 msNormal class: 70619 ms

如您所见，使用此方法大约有10％的速度罚款，因此可以决定应用程序的成本/收益。对我来说，增加的表现力弥补了费用，因为我的申请不会接近很多操作。

除了 MyContext，其他一些课程从原始帖子中更改，我还扩展了控制台测试以涵盖所有用例，因此我在下面发布了完整的解决方案。

class Program{    static void Main(string[] args)    {        var id = 0;        using (var dbContext = new MyContext())        {            var department = new Department();            department.AddFaculty(FacultyEnum.Eng);            department.AddFaculty(FacultyEnum.Math);            dbContext.Department.Add(department);            var department2 = new Department();            department2.AddFaculty(FacultyEnum.Math);            dbContext.Department.Add(department2);            dbContext.SaveChanges();            id = department.Id;        }        using (var dbContext = new MyContext())        {            var department = dbContext.Department.Find(id);            department.AddFaculty(FacultyEnum.Eco);            dbContext.SaveChanges();        }        using (var dbContext = new MyContext())        {            var department = dbContext.Department.Find(id);            var faculty = department.Faculties.Where(x => x.Id == (int)FacultyEnum.Eng).FirstOrDefault();            department.Faculties.Remove(faculty);            dbContext.SaveChanges();        }        using (var dbContext = new MyContext())        {            var department = dbContext.Department.Find(id);            Console.WriteLine($"Department Id {department.Id} has these faculties:");            foreach (var faculty in department.Faculties)            {                Console.WriteLine($"- {faculty.Id}");            }        }        Console.ReadKey();    }}public class MyContext : DbContext{    public DbSet<Department> Department { get; set; }    public DbSet<Faculty> Faculty { get; set; }    public MyContext()        : base(nameOrConnectionString: GetConnectionString())    {        Database.SetInitializer(new MyDbInitializer());    }    public override int SaveChanges()    {        CleanUpFaculties();        return base.SaveChanges();    }    private void CleanUpFaculties()    {        var departments = ChangeTracker            .Entries<Department>()            .Select(x => x.Entity)            .ToList();        var cachedDataToReload = departments            .Select(department => new            {                Department = department,                FacultyIds = department.Faculties.Select(faculty => faculty.Id).ToList(),            })            .ToList();        CleanUpFacultiesOnChangeTracker();        foreach (var item in cachedDataToReload)        {            var faculties = LoadFacultiesFromDb(item.FacultyIds);            typeof(Department).GetProperty("Faculties")                .SetValue(item.Department, faculties);        }    }    private void CleanUpFacultiesOnChangeTracker()    {        var changedEntries = ChangeTracker.Entries<Faculty>().Where(x => x.State != EntityState.Unchanged).ToList();        foreach (var entry in changedEntries)        {            switch (entry.State)            {                case EntityState.Modified:                    entry.CurrentValues.SetValues(entry.OriginalValues);                    entry.State = EntityState.Unchanged;                    break;                case EntityState.Added:                    entry.State = EntityState.Detached;                    break;                case EntityState.Deleted:                    entry.State = EntityState.Unchanged;                    break;            }        }    }    private ICollection<Faculty> LoadFacultiesFromDb(IEnumerable<FacultyEnum> facultyIds)    {        var destination = new List<Faculty>();        foreach (var id in facultyIds)        {            var newFaculty = ChangeTracker                .Entries<Faculty>()                .Where(x => x.State == EntityState.Unchanged && x.Entity.Id == id)                .FirstOrDefault()                ?.Entity;            if (newFaculty == null)            {                newFaculty = Set<Faculty>().Find(id) ?? id;            }            destination.Add(newFaculty);        }        return destination;    }    protected override void OnModelCreating(DbModelBuilder modelBuilder)    {        modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();        modelBuilder.Conventions.Remove<OneToManyCascadeDeleteConvention>();        modelBuilder.Conventions.Remove<ManyToManyCascadeDeleteConvention>();        modelBuilder.Properties<string>()            .Configure(p => p.HasMaxLength(100));        modelBuilder.Configurations.Add(new DepartmentConfiguration());        modelBuilder.Configurations.Add(new FacultyConfiguration());        base.OnModelCreating(modelBuilder);    }    private static string GetConnectionString()    {        return @"Data Source=(localdb)\MSSQLLocalDB;Initial Catalog=TestEnum;Integrated Security=True;Connect Timeout=30;Encrypt=False;TrustServerCertificate=True;ApplicationIntent=ReadWrite;MultiSubnetFailover=False;MultipleActiveResultSets=true;";    }}public class MyDbInitializer : DropCreateDatabaseIfModelChanges<MyContext>{    protected override void Seed(MyContext context)    {        context.Faculty.SeedEnumValues<Faculty, FacultyEnum>(theEnum => theEnum);        context.SaveChanges();        base.Seed(context);    }}public class DepartmentConfiguration : EntityTypeConfiguration<Department>{    public DepartmentConfiguration()    {        HasMany(x => x.Faculties)            .WithMany();    }}public class FacultyConfiguration : EntityTypeConfiguration<Faculty>{    public FacultyConfiguration()    {        Property(x => x.Id)            .HasDatabaseGeneratedOption(DatabaseGeneratedOption.None);    }}public class Department{    public int Id { get; private set; }    public virtual ICollection<Faculty> Faculties { get; private set; }    public Department()    {        Faculties = new List<Faculty>();    }    public void AddFaculty(FacultyEnum faculty)    {        Faculties.Add(faculty);    }}public class Faculty{    public FacultyEnum Id { get; private set; }    public string Name { get; private set; }    public string Description { get; private set; }    private Faculty(FacultyEnum theEnum)    {        Id = theEnum;        Name = theEnum.ToString();        Description = theEnum.Description();    }    protected Faculty() { } //For EF    public static implicit operator Faculty(FacultyEnum theEnum) => new Faculty(theEnum);    public static implicit operator FacultyEnum(Faculty faculty) => faculty.Id;}public enum FacultyEnum{    [Description("English")]    Eng,    [Description("Mathematics")]    Math,    [Description("Economy")]    Eco,}public static class Extensions{    public static string Description<TEnum>(this TEnum item)        => item.GetType()               .GetField(item.ToString())               .GetCustomAttributes(typeof(DescriptionAttribute), false)               .Cast<DescriptionAttribute>()               .FirstOrDefault()?.Description ?? string.Empty;    public static void SeedEnumValues<T, TEnum>(this IDbSet<T> dbSet, Func<TEnum, T> converter)        where T : class => Enum.GetValues(typeof(TEnum))                               .Cast<object>()                               .Select(value => converter((TEnum)value))                               .ToList()                               .ForEach(instance => dbSet.AddOrUpdate(instance));}

输出将是：

// Department Id 1 has these faculties: // - Math// - Eco