Entity Framework主从表数据加载方式

2022-8-12 08:49| 发布者: 江湖百晓生| 查看: 479| 评论: 0

摘要: 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 ...

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);
}
}

到此这篇关于Entity Framework主从表数据加载方式的文章就介绍到这了。希望对大家的学习有所帮助，也希望大家多多支持脚本之家。