EntityFramework 学习 一 Update Entity Graph using DbContext:

使用主键属性

每个实体必须有主键

默认值的id属性值必须为0

 在context2中，它不知道实体的状态，

只能通过实体的主键来判断实体的状态

如果主键为0，则是新的对象，不为0 就是修改

Standard disconnectedStandard = null;

using (var context = new SchoolDBEntities())
{
    context.Configuration.ProxyCreationEnabled = false;

    disconnectedStandard = context.Standards.Where(s => s.StandardId == 58).Include(s => s.Teachers).FirstOrDefault<Standard>();
}
//Update Standard in disconnected mode
disconnectedStandard.StandardName = "Edited Standard Name";
            
//Update teachers collection by editing first teacher and adding new teacher
disconnectedStandard.Teachers.ElementAt(0).TeacherName = "Edited Teacher Name";
disconnectedStandard.Teachers.Add(new Teacher() { TeacherName = "New Teacher", StandardId = disconnectedStandard.StandardId });

using (var newContext = new SchoolDBEntities())
{
    //mark standard based on StandardId
    newContext.Entry(disconnectedStandard).State = disconnectedStandard.StandardId == 0 ? EntityState.Added : EntityState.Modified;

    //mark teacher based on StandardId
    foreach (Teacher tchr in disconnectedStandard.Teachers)
        newContext.Entry(tchr).State = tchr.TeacherId == 0 ? EntityState.Added : EntityState.Modified;
                
                
    newContext.SaveChanges();
}

使用主键的好处有：

1.不需要多余的代码来决定实体的状态

2.良好的性能

不好的地方有 ;

1.每个实体都需要有主键，没有主键就不能确定实体的状态

2.不能决定unchanged状态的实体，如果实体没有改变也会被设置为modified状态，这就没必要去更新没有改变的实体

3.不能删除实体

方法2：

设置实体的State属性

首先，创建一个带有枚举参数的接口

interface IEntityObjectState
{
    EntityObjectState ObjectState { get; set; }
}

public enum EntityObjectState
{ 
    Added,
    Modified,
    Deleted,
    Unchanged
}

 其次，每个实体都实现该接口

public partial class Standard:IEntityObjectState
{
    public Standard()
    {
        this.Students = new HashSet<Student>();
        this.Teachers = new HashSet<Teacher>();
    }
    
    public int StandardId { get; set; }
    public string StandardName { get; set; }
    public string Description { get; set; }
    
    public virtual ICollection<Student> Students { get; set; }
    public virtual ICollection<Teacher> Teachers { get; set; }
    [NotMapped]
    public EntityObjectState ObjectState
    {
        get;
        set;
    }
}

public partial class Teacher:IEntityObjectState
{
    public Teacher()
    {
        this.Courses = new HashSet<Course>();
    }
    
    public int TeacherId { get; set; }
    public string TeacherName { get; set; }
    public Nullable<int> StandardId { get; set; }
    
    public virtual ICollection<Course> Courses { get; set; }
    public virtual Standard Standard { get; set; }

    [NotMapped]
    public EntityObjectState ObjectState
    {
        get;
        set;
    }

}

在客户端设置实体的状态

Teacher existingTeacher = null;

using (var context = new SchoolDBEntities())
{
    context.Configuration.ProxyCreationEnabled = false;
    existingTeacher = context.Teachers.FirstOrDefault<Teacher>();

}
Standard disconnectedStandard = new Standard() { StandardName = "New Standard", ObjectState = EntityObjectState.Added };
existingTeacher.ObjectState = EntityObjectState.Modified;
//add existing teacher(in db) to standard
disconnectedStandard.Teachers.Add(existingTeacher);
//add new standard
disconnectedStandard.Teachers.Add(new Teacher() { TeacherName = "New teacher", StandardId = disconnectedStandard.StandardId, ObjectState = EntityObjectState.Added });
            

using (var newContext = new SchoolDBEntities())
{
    //check the ObjectState property and mark appropriate EntityState 
    if (disconnectedStandard.ObjectState == EntityObjectState.Added)
        newContext.Entry(disconnectedStandard).State = System.Data.Entity.EntityState.Added;
    else if (disconnectedStandard.ObjectState == EntityObjectState.Modified)
        newContext.Entry(disconnectedStandard).State =System.Data.Entity.EntityState.Modified;
    else if (disconnectedStandard.ObjectState == EntityObjectState.Deleted)
        newContext.Entry(disconnectedStandard).State = System.Data.Entity.EntityState.Deleted;
    else
        newContext.Entry(disconnectedStandard).State = System.Data.Entity.EntityState.Unchanged;

    //check the ObjectState property of each teacher and mark appropriate EntityState 
    foreach (Teacher tchr in disconnectedStandard.Teachers)
    {
        if (tchr.ObjectState == EntityObjectState.Added)
            newContext.Entry(tchr).State = System.Data.Entity.EntityState.Added;
        else if (tchr.ObjectState == EntityObjectState.Modified)
            newContext.Entry(tchr).State = System.Data.Entity.EntityState.Modified;
        else if (tchr.ObjectState == EntityObjectState.Deleted)
            newContext.Entry(tchr).State = System.Data.Entity.EntityState.Deleted;
        else
            newContext.Entry(tchr).State = System.Data.Entity.EntityState.Unchanged;
    }
    //save changes
    newContext.SaveChanges();
}