Entity Framework（04）：主从表数据加载方式

一、延迟加载：LazyLoading

使用延迟加载，关联的实体必须标注为virtual。

本例是标注Destination类里的Lodgings为virtual。因为先发sql去查询主键对象，然后根据主键id去从表里查相关联的数据。

private static void TestLazyLoading()
    {
        using (var context = new CodeFirst.DataAccess.BreakAwayContext())
        {
           var canyon = (from d in context.Destinations where d.Name == "Grand Canyon" select d).Single();

           var distanceQuery = from l in canyon.Lodgings   //延迟加载canyon的所有.Lodgings
                                where l.Name == "HuangShan Hotel"
                                select l;
            foreach (var lodging in distanceQuery)
                Console.WriteLine(lodging.Name);
        }
    }

改进：在数据库中操作,显示加载

private static void QueryLodgingDistancePro()
    {
        using (var context = new CodeFirst.DataAccess.BreakAwayContext())
        {
            var canyon = (from d in context.Destinations where d.Name == "Grand Canyon" select d).Single();
            var lodgingQuery = context.Entry(canyon).Collection(d => d.Lodgings).Query();//接下来的查询在数据库中，包括Count（）等
            var distanceQuery = from l in lodgingQuery 

                                 where l.Name == "HuangShan Hotel" 

                                 select l;

            foreach (var lodging in distanceQuery)
                Console.WriteLine(lodging.Name);
        }
    }

二、贪婪加载：EagerLoading

private static void TestEagerLoading()
    {
        using (var context = new CodeFirst.DataAccess.BreakAwayContext())
        {
            // var allDestinations = context.Destinations.Include(d => d.Lodgings);
            var AustraliaDestination = context.Destinations.Include(d => d.Lodgings).Where(d => d.Name == "Bali");
            //context.Lodgings.Include(l => l.PrimaryContact.Photo);
            //context.Destinations.Include(d => d.Lodgings.Select(l => l.PrimaryContact));
            //context.Lodgings.Include(l => l.PrimaryContact).Include(l => l.SecondaryContact);
            foreach (var destination in AustraliaDestination)
            {
                foreach (var lodging in destination.Lodgings)
                    Console.WriteLine(" - " + lodging.Name);
            }
        }
    }

三、显示加载：ExplicitLoading

1、查找导航属性为一个集合的

private static void LoadRelateData()
    {
        using (var context = new CodeFirst.DataAccess.BreakAwayContext())
        {
           var canyon = (from d in context.Destinations where d.Name == "Grand Canyon" select d).Single();

           context.Entry(canyon).Collection(d => d.Lodgings).Load();  //显示加载

           foreach (var lodging in context.Lodgings.Local)
                Console.WriteLine(lodging.Name);
        }
    }

2、查找导航属性为一个实体对象的

private static void LoadPrimaryKeyData()
    {
        using (var context = new CodeFirst.DataAccess.BreakAwayContext())
        {
            var lodging = context.Lodgings.First();

            context.Entry(lodging).Reference(l => l.Destination).Load();

           foreach (var destination in context.Destinations.Local)   //遍历的是内存中的Destinations数据
                Console.WriteLine(destination.Name);
        }
    }