Entity Framework 6 Recipes 2nd Edition（13-3）译 -> 为一个只读的访问获取实体

问题

你想有效地获取只是用来显示不会更新的操作的实体.另外,你想用CodeFirst的方式来实现

解决方案

一个非常常见行为,尤其是网站,就是只是让用户浏览数据.大多数情况下,用户不会更新数据.在这种情况下,你可以通过避开上下文的缓存和修改跟踪来提高代码性能,你可以非常简单地使用AsNoTracking方法来实现.

让我们假设你一个应用程序来管理doctor(医生)的appointments(预约),你的模型如下图Figure 13-5.

 

Figure 13-5. A model for managing doctors and their appointments

 

         首先,这个例子用EF的CodeFirst方式来实现.,在Listing 13-6创建我们的实体类, Company, Doctor, 和Appointment.

 

Listing 13-6. The Company, Doctor,and Appointment.Entity Object

   public class Company

    {

        public Company()

        {

            Doctors = new HashSet<Doctor>();

        }

        public int CompanyId { get; set; }

        public string Name { get; set; }

        public virtual ICollection<Doctor> Doctors { get; set; }

    }

    public class Doctor

    {

        public Doctor()

        {

            Appointments = new HashSet<Appointment>();

        }

        public int DoctorId { get; set; }

        public string Name { get; set; }

        public int CompanyId { get; set; }

        public virtual ICollection<Appointment> Appointments { get; set; }

        public virtual Company Company { get; set; }

    }

    public class Appointment

    {

        public int AppointmentId { get; set; }

        public System.DateTime AppointmentDate { get; set; }

        public string Patient { get; set; }

        public int DoctorId { get; set; }

        public virtual Doctor Doctor { get; set; }

}

接下来,在Listing 13-7,我们创建用CodeFirst方式时访问EF的途径,DbContext对象

Listing 13-7. DbContext Object

 

    public class Recipe3Context : DbContext

    {

        public Recipe3Context()

            : base("Recipe3ConnectionString")

        {

            // Disable Entity Framework Model Compatibility

            Database.SetInitializer<Recipe3Context>(null);

        }

 

        protected override void OnModelCreating(DbModelBuilder modelBuilder)

        {

            modelBuilder.Entity<Appointment>().ToTable("Chapter13.Appointment");

            modelBuilder.Entity<Company>().ToTable("Chapter13.Company");

            modelBuilder.Entity<Doctor>().ToTable("Chapter13.Doctor");

        }

        public DbSet<Appointment> Appointments { get; set; }

        public DbSet<Company> Companies { get; set; }

        public DbSet<Doctor> Doctors { get; set; }

    }

接下来我们在项目中添加App.Config,并把下列Listing 13-8的代码加入到ConnectionString节下

Listing 13-8. Connection String

<connectionStrings>

<add name="Recipe3ConnectionString"

connectionString="Data Source=.;

Initial Catalog=EFRecipes;

Integrated Security=True;

MultipleActiveResultSets=True"

providerName="System.Data.SqlClient" />

</connectionStrings>

为了获取Doctors和Companies并使他们不添加到上下文对象中,我们把AsNoTracking方法链接到获取实体的查询中,如Listing 13-9 那样.

Listing 13-9. Doing a Simple Query Using the AsNoTracking Method

   using (var context = new Recipe3Context())

            {

                var company = new Company { Name = "Paola Heart Center" };

                var doc1 = new Doctor { Name = "Jill Mathers", Company = company };

                var doc2 = new Doctor { Name = "Robert Stevens", Company = company };

                var app1 = new Appointment

                {

                    AppointmentDate = DateTime.Parse("3/18/2010"),

                    Patient = "Karen Rodgers",

                    Doctor = doc1

                };

                var app2 = new Appointment

                {

                    AppointmentDate = DateTime.Parse("3/20/2010"),

                    Patient = "Steven Cook",

                    Doctor = doc2

                };

                context.Doctors.Add(doc1);

                context.Doctors.Add(doc2);

                context.Appointments.Add(app1);

                context.Appointments.Add(app2);

                context.Companies.Add(company);

                context.SaveChanges();

            }

            using (var context = new Recipe3Context())

            {

                Console.WriteLine("Entities tracked in context for Doctors...");

                // 用AsNoTracking() 方法执行查询

                context.Doctors.Include("Company").AsNoTracking().ToList();

                Console.WriteLine("Number of entities loaded into context with AsNoTracking: {0}",

                context.ChangeTracker.Entries().Count());//输出:0

                // 不用AsNoTracking() 方法执行查询

                context.Doctors.Include("Company").ToList();

                Console.WriteLine("Number of entities loaded into context without AsNoTracking: {0}",

                context.ChangeTracker.Entries().Count());//输出:3

            }

 

输出结果如下:

Entities tracked in context for Doctors...

Number of entities loaded into context with AsNoTracking: 0

Number of entities loaded into context without AsNoTracking: 3

 

它是如何工作的

当我们把AsNoTracking方法链接上你的查询,从查询返回的结果不会被上下文跟踪.在我们的例子中,我们显示地Include了Doctor的Comanpies.

默认情况下,你的查询结果会被上下文跟踪,这使得更新和删除更容易,但是代价是付出更多的内存和CPU负载.为应用程序串连更多的对象,比如在电子商务网站上浏览产品,使用AsNoTracking选项,能节省更多的资源,使应用程序性能更高.

         没有缓存一个查询结果,你每次都得为查询实例化.通常地,使修改跟踪可用,EF将不需要为一个上下文中已经存在的实例再实例化.

         当你包含AsNoTracking选项(如我们的Listing 13-9),它只影响当前查询的实体.它不会影响后面的不包含AsNoTracking选项的查询,如Listing 13-9 演示的