EF Core 中使用无连接跟踪图

在上一篇文章中，学习了在有连接场景中EF Core的ChangeTracker如何自动更改EntityState。在本篇文章中，我们将学习在Entity Framework Core中无连接方案中的根实体和子实体上的不同方法的行为和跟踪图。

实体框架核心提供以下不同的方法，它们不仅将实体附加到上下文，还更改EntityState无连接的实体图中的每个实体：

• Attach()
• Entry()
• Add()
• Update()
• Remove()

让我们看看上述方法用Entity Framework Core 2.x中的跟踪图如何中更改每个实体的 EntityState 。

Attach()方法

DbContext.Attach()和DbSet.Attach()方法附加指定无连接的实体图，并开始跟踪它。它们返回一个实例EntityEntry，用于分配适当的实例EntityState。

以下示例演示了DbContext.Attach()方法对EntityState图中每个实体的行为。

public static void Main()
{
    var stud = new Student() { //Root entity (empty key)
        Name = "Bill",
        Address = new StudentAddress()  //Child entity (with key value)
        {
            StudentAddressId = 1,
            City = "Seattle",
            Country = "USA"
        },
        StudentCourses = new List<StudentCourse>() {
            new StudentCourse(){  Course = new Course(){ CourseName = "Machine Language" } },//Child entity (empty key)
            new StudentCourse(){  Course = new Course(){  CourseId = 2 } } //Child entity (with key value)
        }
    };

    var context = new SchoolContext();
    context.Attach(stud).State = EntityState.Added;  

    DisplayStates(context.ChangeTracker.Entries());
}

private static void DisplayStates(IEnumerable<EntityEntry> entries)
{
    foreach (var entry in entries)
    {
        Console.WriteLine($"Entity: {entry.Entity.GetType().Name},
                             State: {entry.State.ToString()} ");
    }
}

Output:
Entity: Student, State: Added 
Entity: StudentAddress, State: Unchanged 
Entity: StudentCourse, State: Added 
Entity: Course, State: Added 
Entity: StudentCourse, State: Added 
Entity: Course, State: Unchanged

在上面的示例中，stud是Student实体图的实例，其包括StudentAddress和StudentCourse实体的引用。 context.Attach(stud).State = EntityState.Added将stud实体图附加到上下文并为其设置已添加状态。

的Attach()添加方法设置EntityState到根实体（在这种情况下，Student不管其是否包含密钥值或没有）。如果子实体包含键值，则它将被标记为未更改，否则将标记为已添加。上面示例的输出显示Student实体已添加EntityState，具有非空键值的子实体具有未更改EntityState，具有空键值的子实体具有已添加状态。

下表列出了Attach()在设置不同EntityState的断开连接的实体图时方法的行为。

Entry()方法

DbContext.Entry()与先前的EF 6.x相比， 该方法在Entity Framework Core中的行为有所不同。请考虑以下示例：

Output:
Entity: Student, State: Modified

在上面的示例中，context.Entry(student).State = EntityState.Modified将实体附加到上下文并将指定的EntityState（在本例中为Modified）应用于根实体，而不管它是否包含Key属性值。它会忽略图表中的所有子实体，并且不会附加或设置它们EntityState。

下表列出了该DbContext.Entry()方法的不同行为。

Add()方法

该DbContext.Add和DbSet.Add方法附加实体图来添加上下文并设置EntityState到根和子实体，不论他们是否有键值与否。

var student = new Student() { //Root entity (with key value)
    StudentId = 1,
    Name = "Bill",
    Address = new StudentAddress()  //Child entity (with key value)
    {
        StudentAddressId = 1,
        City = "Seattle",
        Country = "USA"
    },
    StudentCourses = new List<StudentCourse>() {
            new StudentCourse(){  Course = new Course(){ CourseName="Machine Language" } },//Child entity (empty key)
            new StudentCourse(){  Course = new Course(){  CourseId=2 } } //Child entity (with key value)
        }
};
var context = new SchoolContext();
context.Students.Add(student);
DisplayStates(context.ChangeTracker.Entries());

Output:
Entity: Student, State: Added 
Entity: StudentAddress, State: Added
Entity: StudentCourse, State: Added 
Entity: Course, State: Added 
Entity: StudentCourse, State: Added 
Entity: Course, State: Added

下表列出了使用DbContext.Add或DbSet.Add方法的图中每个实体的可能EntityState 。

Update()方法

DbContext.Update()和DbSet.Update()方法附加的实体图形到上下文，并设置EntityState这取决于它是否包含一个键属性值或者未每个实体中的曲线图。请考虑以下示例。

var student = new Student() { //Root entity (with key value)
    StudentId = 1,
    Name = "Bill",
    Address = new StudentAddress()  //Child entity (with key value)
    {
        StudentAddressId = 1,
        City = "Seattle",
        Country = "USA"
    },
    StudentCourses = new List<StudentCourse>() {
            new StudentCourse(){  Course = new Course(){ CourseName="Machine Language" } },//Child entity (empty key)
            new StudentCourse(){  Course = new Course(){  CourseId=2 } } //Child entity (with key value)
        }
};
var context = new SchoolContext();
context.Update(student);
DisplayStates(context.ChangeTracker.Entries());

Output:
Entity: Student, State: Modified 
Entity: StudentAddress, State: Modified 
Entity: StudentCourse, State: Added 
Entity: Course, State: Added 
Entity: StudentCourse, State: Added 
Entity: Course, State: Modified

在上面的示例中，该Update()方法将Modified状态应用于包含非空键属性值的实体，并将Added状态应用于包含空或默认CLR键值的实体，而不管它们是根实体还是子实体。

Remove()方法

该DbContext.Remove()和DbSet.Remove()方法已删除设置EntityState到根实体。

var student = new Student() { //Root entity (with key value)
    StudentId = 1,
    Name = "Bill",
    Address = new StudentAddress()  //Child entity (with key value)
    {
        StudentAddressId = 1,
        City = "Seattle",
        Country = "USA"
    },
    StudentCourses = new List<StudentCourse>() {
            new StudentCourse(){  Course = new Course(){ CourseName="Machine Language" } },//Child entity (empty key)
            new StudentCourse(){  Course = new Course(){  CourseId=2 } }
        }
};
var context = new SchoolContext();
context.Remove(student);
DisplayStates(context.ChangeTracker.Entries());

Output:
Entity: Student, State: Deleted 
Entity: StudentAddress, State: Unchanged
Entity: StudentCourse, State: Added 
Entity: Course, State: Added 
Entity: StudentCourse, State: Added 
Entity: Course, State: Unchanged

下表列出了每个实体Remove()上方法的行为EntityState。

因此，在EF Core中使用上述方法时要小心。

ChangeTracker.TrackGraph()在下一章中 了解处理实体图中每个实体的方法。