From 024ae332a43f85d342ee816b01a9e2ccddc397c4 Mon Sep 17 00:00:00 2001
From: AlexLEWIS <liuyis@live.com>
Date: Wed, 29 Sep 2021 11:15:11 +0800
Subject: [PATCH] Created Greed Loading (markdown)

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+[中文](%e8%b4%aa%e5%a9%aa%e5%8a%a0%e8%bd%bd) | **English**
+
+## 1、导航属性 ManyToOne
+
+ManyToOne 导航属性通过 ToList(includeNestedMembers: false) 加载，参数说明：
+
+false: 返回 2级 Join 的导航数据（默认）；
+
+true: 返回所有层级深度 Join 的导航数据（未使用的导航数据不会返回）；
+
+```csharp
+Select<Tag>().Include(a => a.Parent.Parent).ToList(true);
+
+Select<Tag>().Where(a => a.Parent.Parent.Name == "1").ToList(true);
+//这样写，不需要再标记 Join，解析表达式时自动处理成 LeftJoin
+```
+
+## 2、导航属性 OneToMany/ManyToMany
+
+IncludeMany 贪婪加载集合的导航属性，其实是分两次查询，在 ToList 后进行了数据重装。
+
+```csharp
+Select<Tag>().IncludeMany(a => a.Songs).ToList();
+```
+
+IncludeMany 有第二个参数，可以进行二次查询前的修饰工作。
+
+```csharp
+Select<Tag>().IncludeMany(a => a.Songs, 
+    then => then.Where(song => song.User == "admin")).ToList();
+```
+
+其实在 then 那里，还可以继续进行向下 Include/IncludeMany。只要你喜欢，向下 100 层都没问题。
+
+## 3、变异
+
+没有配置导航关系，也可以贪婪加载。
+
+```csharp
+Select<Tag>().IncludeMany(a => a.TestManys.Where(b => b.TagId == a.Id));
+```
+
+只查询每项子集合的前几条数据，避免像EfCore加载所有数据导致IO性能低下（比如某商品下有2000条评论）。
+
+```csharp
+Select<Tag>().IncludeMany(a => a.TestManys.Take(10));
+```
+
+子集合返回部分字段，避免字段过多的问题。
+
+```csharp
+Select<Tag>().IncludeMany(a => a.TestManys.Select(b => new TestMany { Title = b.Title ... }));
+```
+
+## 4、IncludeMany 扩展方法
+
+当主数据已存在内存中，子数据怎么加载？所以我们增加了 List\<T\> 扩展方法，示例如下：
+
+```csharp
+new List<Song>(new[] { song1, song2, song3 }).IncludeMany(fsql, a => a.Tags);
+```
+
+## 5、IncludeMany 两种方式对比
+
+方式一（IncludeMany 扩展方法）：
+
+```csharp
+var list111 = fsql.Select<SysModule>()
+    .Page(1, 10)
+    .ToList(a => new { Id = a.Id }) //查询数据 id
+    .Select(a => new SysModule { Id = a.Id }).ToList() //内存操作
+    .IncludeMany(fsql, a => a.Permissions, then => then.Include(a => a.Button));
+```
+
+```sql
+SELECT a."Id" as1 
+FROM "SysModule" a 
+limit 0,10
+
+SELECT a."Id", a."SysModuleId", a."SysModuleButtonId", a."Status", 
+a__Button."Id" as5, a__Button."Name", a__Button."EventName", a__Button."EnCode", a__Button."Icon", a__Button."Sort", a__Button."CreateTime" 
+FROM "SysModulePermission" a 
+LEFT JOIN "SysModuleButton" a__Button ON a__Button."Id" = a."SysModuleButtonId" 
+WHERE ((a."SysModuleId") in ('menu1','menu2'))
+```
+
+---
+
+方式二（直接 IncludeMany + ToList）：
+
+```csharp
+var list222 = fsql.Select<SysModule>()
+    .IncludeMany(m => m.Permissions, then => then.Include(a => a.Button))
+    .Page(1, 10)
+    .ToList();
+```
+
+```sql
+SELECT a."Id", a."ParentId", a."Name", a."Icon", a."UrlAddress", a."IsShow", a."Sort", a."Description", a."CreateTime" 
+FROM "SysModule" a 
+limit 0,10
+
+SELECT a."Id", a."SysModuleId", a."SysModuleButtonId", a."Status", 
+a__Button."Id" as5, a__Button."Name", a__Button."EventName", a__Button."EnCode", a__Button."Icon", a__Button."Sort", a__Button."CreateTime" 
+FROM "SysModulePermission" a 
+LEFT JOIN "SysModuleButton" a__Button ON a__Button."Id" = a."SysModuleButtonId" 
+WHERE ((a."SysModuleId") in ('menu1','menu2'))
+```
+
+案例：查询 Vod 表，分类1、分类2、分类3 各10条数据
+
+```csharp
+class Vod {
+    public Guid Id { get; set; }
+    public int TypeId { get; set; }
+}
+
+//定义临时类，也可以是 Dto 类
+class Dto {
+    public int TypeId { get; set; }
+    public List<Vod> Vods { get; set; }
+}
+
+var dto = new [] { 1,2,3 }.Select(a => new Dto { TypeId = a }).ToList();
+dto.IncludeMany(fsql, d => d.Vods.Take(10).Where(vod => vod.TypeId == d.TypeId));
+
+//执行后，dto 每个元素.Vods 将只有 10条记录
+```
+
+## Reference
+
+- [《Query from Multi Tables》](Query-from-Multi-Tablea)
+- [《Return Data》](Return-Data)
+- [《LinqToSql》](Linq-to-Sql)
+- [《Repository Layer》](Repository-Layer)
+- [《Filters and Global Filters》](Filters-and-Global-Filters)
+- [《FreeSql Optimization: Lazy Loading》](Lazy-Loading)
+- [《FreeSql Optimization: Greed Loading》](Greed-Loading)
+- [《Expression Function》](Expression-Function)
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