【C语言教程】Entity Framework主从表数据加载方式详解

所需工具：

C++

聪明的大脑

勤劳的双手

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教程如下

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);
 	         }
 	     }