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();
}
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