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); } }
posted on 2020-07-03 14:03 springsnow 阅读(232) 评论(0) 编辑 收藏 举报