EF的三种数据加载方式

Posted 2020-08-04

EF中有3种数据加载方式，具体如下：

延迟加载（默认）： 设置导航属性为virtual

贪婪加载：不设置导航属性为virtual，并且对导航属性使用Include

显示加载：不设置导航属性为virtual，并且对导航属性使用Reference(单个对象).Load()或Collection(对象集).Load()

    public class Programm
    {
        public static void Main()
        {
            TestDbContext db = new TestDbContext();

            //延迟加载
            var res1 = db.Reservations;
            foreach (var item in res1)
            {
                //do something
            }

            //贪婪加载
            //有多个Include会全部完成加载，生成的sql命令是一条完成的
            var res2 = db.Reservations.Include(r => r.Details);

            //显示加载
            var res3 = db.Reservations.ToList();
            foreach (var item in res3)
            {
                var temp = db.Entry(item);
                temp.Collection(i => i.Details).Load();
                temp.Reference(i => i.Customer).Load();
            }
        }
    }

    public class Reservation
    {
        public int ReservationId { get; set; }
        public string ClientName { get; set; }
        public string Location { get; set; }
        public List<ReservationDetails> Details { get; set; }
        public Customer Customer { get; set; }
    }

    public class ReservationDetails
    {
        public int Id { get; set; }
        public DateTime Time { get; set; }
    }

    public class Customer
    {
        public int Id { get; set; }
        public string Name { get; set; }
    }

    public class TestDbContext : DbContext
    {
        public DbSet<Reservation> Reservations { get; set; }
    }