关于前端:EF关联对象加载模式

EF 默认不会加载 entity 所关联的 entity（navigation properties），比方学生（Student）床铺（SleepIn）的地位（Location）：

Student zl = context.Students.Where(s => s.Name == “zl”).Single(); Console.WriteLine(zl.SleepInId); //OK Console.WriteLine(zl.SleepIn.Location); //NullReference Exception

否则因为 entity 之间大量的、链式的、简单的援用关系，会导致一次性的加载大量 entity，耗尽零碎内存资源。

但如果所有的关联 entity 要靠开发人员本人依据其 Id 查问获取，又显得累赘，所以 EF 提供了三种加载关联数据的模式：

关联的数据会随着 entity 加载同时加载。这须要用到办法：

Include()：蕴含以后 entity 的关联对象，和
ThenInclude()：蕴含以后 entity 关联对象的关联对象，通常用于关联_汇合_entity 下的再一次关联。
晚期版本的 VSThenInclude()的智能提醒可能有问题，略过谬误提醒间接运行即可
Teacher fg = context.Teachers.Where(t => t.Name == “fg”) // 找到 Teacher .Include(t => t.Students) // 蕴含 Teacher 的 Students 汇合，.ThenInclude(s => s.StudyIn) // 再蕴含 Student 的 StudyIn（Classroom）.SingleOrDefault();

如果 Student 不是汇合，能够始终点 … 比方：

Teacher fg = context.Teachers.Where(t => t.Name == “fg”) .Include(t => t.Major.Bases) //Major 和 Bases 都是 entity

另外，

Include()能够间接并用，即一个 entity 下 Include()多个 Property，但
ThenInclude()不行_（编译器无奈辨别）_

所以，如果须要同时 ThenClude()多个 Pproperty，不能

Teacher fg = context.Teachers.Where(t => t.Name == “fg”) .Include(t => t.Students) .ThenInclude(s => s.StudyIn) .ThenInclude(s => s.SleepIn)

只能：

Teacher fg = context.Teachers.Where(t => t.Name == “fg”) .Include(t => t.Students) .ThenInclude(s => s.StudyIn) .Include(t => t.Students) // 看似反复，但必须这样写 .ThenInclude(s => s.SleepIn)

演示：

F9 断点，Include()之后能看到关联 entity 的值
logger，生成的 SQL 语句__应用 JOIN，一次性获取全副数据

一旦被加载，就会被 DbContext 缓存 / 追踪（track）
如果关联数据此前曾经被缓存 / 追踪，那么即便你没有 Include()，一样能够被主动关联应用
Bed bed = context.Find<Bed>(1); // 实现曾经将 Id= 1 的 Bed 加载到 DbContext 中 var query = context.Students.Where(s => s.Name == “zl”) //.Include(s => s.SleepIn) 这里没有 Include 学生的床铺
如果最初的后果（Selcet 语句）没有应用到 Include()里的值，Include 会被 ignore
var query = context.Students.Where(s => s.Name == “zl”) .Include(s => s.SleepIn) .Select(s => new { s.Name}); // 只取 Student.Name，没用到 Student.SleepIn

Include 中能够强制转换（多态）：

.ThenInclude(t => ((Teacher)t).TeachOn) .ThenInclude(t => (t as Teacher).TeachOn)

在曾经取到一个 entity 之后，再利用办法：

Entry()：获取该 entity 在 DbContext 中的追踪信息，而后调用其
之前咱们学习过的影子属性，能够用 Entry() 办法获取
```
context.Entry(zl).Property("EnrollDate").CurrentValue = DateTime.Now;
```
Reference()/Collection()办法，表明要获取该 entity 的单个的关联（Reference）或汇合（Collection）对象，最初再调用
Load()：将其加载
context.Entry(zl) .Reference(s => s.StudyIn) .Load(); context.Entry(zl) .Collection(s => s.Teachers) .Load();

演示：

EF 分两次用 SQL 查问别离获取 entity 信息
Load()之后 EF 主动赋值 entity 关联属性值

在.NET core 2.1 之后反对，须要增加援用 Microsoft.EntityFrameworkCore.Proxies，并显式开启：

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder) => optionsBuilder .UseLazyLoadingProxies() .UseSqlServer(myConnectionString);

所波及的 entity 必须是可被继承的，且关联 entity 属性必须是可 override 的（virtual）：

public class Student : Person {public virtual Bed SleepIn { get; set;} public virtual IList<Teacher> Teachers {get; set;} public virtual Classroom StudyIn {get; set;} }

因为，Lazyload 实质上是：

用一个独自的 SQL 语句获取要加载 entity，比方 Student
其关联 entity不是 NULL，而是由 EF 主动新建的一个子类（后缀 Proxy）对象，比方 SleepInProxy
proxy 对象只有主键（Id）被赋值，其余属性值为默认值（null，0，false 等）
然而，当开发人员视图获取关联 entity 时，proxy 对象会主动连贯数据库，获取相干数据，予以加载

演示：

如果是关联的汇合，留神应用 Query()防止全副加载：

studentRep.Entry(ht) .Collection(m => m.Majors) .Query().Where(m => m.MajorId > 3) .Load();

按咱们之前的办法，entity 援用的汇合（collection），都会一次性的把所有汇合都取出来。

如果汇合的数据量比拟大，且咱们不须要汇合的全副信息，这就是一种性能节约。比方，咱们只想取：

汇合元素的个数
汇合的某一部分

EF 为咱们提供了

取出所有教室的所有学生：

foreach (var room in context.Classrooms .ToList() / 必不可少，否则会有 ADO.NET 的 DataReader 谬误 /) {Console.WriteLine(room.Name); foreach (var student in room.Students) {Console.WriteLine($” {student.Name}”); } }

因为 LazyLoad，外个循环里，EF 会取出所有的 Classrooms，但不会取出 Classroom 中的 Student 对象；

内循环里，

补充 Include()

foreach (var room in context.Classrooms.Include(c=>c.Students))

优劣总结：

Lazy Load：应用起来最不便，但不足精细化管制（_至多目前是这样的？_）有可能带来 1 + n 的性能问题
Explicit Load：会产生屡次查问，但能够应用 Query()对关联数据进行 SQL 语句筛选，而且能够重用
Eager Load：应用 join 查问，缩小数据库拜访次数，但不适宜关联大数据的情景（尤其是咱们实际上只须要局部关联数据的时候）

飞哥集体偏好：Lazy 为主，Eager 为附

为什么须要？

而不应用用 query 查问（Join）：保障 entity 不依赖于 repository（database）

没有 ThenInclude，须要 Select

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关于前端:EF关联对象加载模式

事后（Eager）加载

track 和优化

显式（Explicit）加载

提早（惰性 Lazy）加载

性能进步

IQueryable

1+ N 问题

EF6 的不同