1. 基本保存

　　每个DBContext实例都有一个ChangeTracker，负责跟踪需要写入数据库的更改。当实例发生更改时，更改会被记录在ChangeTracker中，在调用 SaveChanges 时被写入数据库。

1.1 添加数据

　　使用 DbSet.Add()添加实体类的新实例。调用 SaveChanges() 时，数据将插入到数据库中。

using (var context = new LibingContext()){    var role = new Role    {        RoleName = "教师"    };    context.Roles.Add(role);    context.SaveChanges();}

1.2 更新数据

　　Entity Framwork Core将自动检测对由DbContext跟踪的实体所做的更改。

　　更新数据：修改属性值，调用 SaveChanges()。

using (var context = new LibingContext()){    var role = context.Roles.Find(1);    role.RoleName = "教师";    context.SaveChanges();}

1.3 删除数据

　　使用 DbSet.Remove() 删除实体类的实例。

　　如果实体已存在于数据库中，则 SaveChanges() 将删除该实体。如果实体尚未保存到数据库（即跟踪为“已添加”），则 SaveChanges() 时，该实体会从上下文中删除且不再插入。

using (var context = new LibingContext()){    var role = context.Roles.Find(1);    context.Roles.Remove(role);    context.SaveChanges();}

1.4 一个SaveChanges 中的多个操作

　　可以将多个添加/更新/删除操作合并到对“SaveChanges”的单个调用。

　　对于大多数数据库提供程序，“SaveChanges”是事务性的。

using (var context = new LibingContext()){    // 新增    context.Roles.Add(new Role { RoleName = "管理员" });    context.Roles.Add(new Role { RoleName = "学生" });    // 修改    var modifyRole = context.Roles.Find(1);    modifyRole.RoleName = "教师";    // 删除    var deleteRole = context.Roles.Where(t => t.RoleID == 2).FirstOrDefault();    context.Roles.Remove(deleteRole);    context.SaveChanges();}

2. 关联数据

2.1 添加新实体的关系图

using (var context = new LibingContext()){                    var category = new Category    {        CategoryName = "手机",        Products = new List
     
       {            new Product { ProductName = "华为", UnitPrice = 2000 },            new Product { ProductName = "小米", UnitPrice = 1000 }        }    };    context.Categories.Add(category);    context.SaveChanges();}

exec sp_executesql N'SET NOCOUNT ON;INSERT INTO [Category] ([CategoryName])VALUES (@p0);SELECT [CategoryID]FROM [Category]WHERE @@ROWCOUNT = 1 AND [CategoryID] = scope_identity();',N'@p0 nvarchar(4000)',@p0=N'手机'

exec sp_executesql N'SET NOCOUNT ON;INSERT INTO [Product] ([CategoryID], [ProductName], [UnitPrice])VALUES (@p0, @p1, @p2);SELECT [ProductID]FROM [Product]WHERE @@ROWCOUNT = 1 AND [ProductID] = scope_identity();',N'@p0 int,@p1 nvarchar(4000),@p2 decimal(4,0)',@p0=1,@p1=N'华为',@p2=2000

exec sp_executesql N'SET NOCOUNT ON;INSERT INTO [Product] ([CategoryID], [ProductName], [UnitPrice])VALUES (@p0, @p1, @p2);SELECT [ProductID]FROM [Product]WHERE @@ROWCOUNT = 1 AND [ProductID] = scope_identity();',N'@p0 int,@p1 nvarchar(4000),@p2 decimal(4,0)',@p0=1,@p1=N'小米',@p2=1000

2.2 添加关联实体

　　如果从已由DbContext跟踪的实体的导航属性中引用新实体，则该实体将插入到数据库中。

using Microsoft.EntityFrameworkCore;

using (var context = new LibingContext()){    var category = context.Categories        .Include(t => t.Products)        .Where(t => t.CategoryID == 1)        .FirstOrDefault();    category.Products.Add(new Product    {        ProductName = "VIVO",        UnitPrice = 1500    });    context.SaveChanges();}

SELECT TOP(1) [t].[CategoryID], [t].[CategoryName]FROM [Category] AS [t]WHERE [t].[CategoryID] = 1ORDER BY [t].[CategoryID]

SELECT [t.Products].[ProductID], [t.Products].[CategoryID], [t.Products].[ProductName], [t.Products].[UnitPrice]FROM [Product] AS [t.Products]INNER JOIN (    SELECT TOP(1) [t0].[CategoryID]    FROM [Category] AS [t0]    WHERE [t0].[CategoryID] = 1    ORDER BY [t0].[CategoryID]) AS [t1] ON [t.Products].[CategoryID] = [t1].[CategoryID]ORDER BY [t1].[CategoryID]

exec sp_executesql N'SET NOCOUNT ON;INSERT INTO [Product] ([CategoryID], [ProductName], [UnitPrice])VALUES (@p0, @p1, @p2);SELECT [ProductID]FROM [Product]WHERE @@ROWCOUNT = 1 AND [ProductID] = scope_identity();',N'@p0 int,@p1 nvarchar(4000),@p2 decimal(4,0)',@p0=1,@p1=N'VIVO',@p2=1500

2.3 更改关系

　　如果更改实体的导航属性，则将对数据库中的外键列进行相应的更改。

using (var context = new LibingContext()){    var category = context.Categories.Find(2);    var product = context.Products.Find(1);    product.Category = category;    context.SaveChanges();}

exec sp_executesql N'SELECT TOP(1) [e].[CategoryID], [e].[CategoryName]FROM [Category] AS [e]WHERE [e].[CategoryID] = @__get_Item_0',N'@__get_Item_0 int',@__get_Item_0=2

exec sp_executesql N'SELECT TOP(1) [e].[ProductID], [e].[CategoryID], [e].[ProductName], [e].[UnitPrice]FROM [Product] AS [e]WHERE [e].[ProductID] = @__get_Item_0',N'@__get_Item_0 int',@__get_Item_0=1

exec sp_executesql N'SET NOCOUNT ON;UPDATE [Product] SET [CategoryID] = @p0WHERE [ProductID] = @p1;SELECT @@ROWCOUNT;',N'@p1 int,@p0 int',@p1=1,@p0=2

2.4 删除关系

　　可以通过将引用导航设置为 null 或从集合导航中删除相关实体来删除关系。

　　默认情况下，对于必选关系，将配置级联删除行为，并将从数据库中删除子实体/依赖实体。对于可选关系，默认情况下不会配置级联删除，但会将外键属性设置为 null。

using (var context = new LibingContext()){    var category = context.Categories        .Include(t => t.Products)        .Where(t => t.CategoryID == 1)        .FirstOrDefault();    //category.Products.Remove(category.Products.FirstOrDefault());    category.Products = null;    context.SaveChanges();}

SELECT TOP(1) [t].[CategoryID], [t].[CategoryName]FROM [Category] AS [t]WHERE [t].[CategoryID] = 1ORDER BY [t].[CategoryID]

SELECT [t.Products].[ProductID], [t.Products].[CategoryID], [t.Products].[ProductName], [t.Products].[UnitPrice]FROM [Product] AS [t.Products]INNER JOIN (    SELECT TOP(1) [t0].[CategoryID]    FROM [Category] AS [t0]    WHERE [t0].[CategoryID] = 1    ORDER BY [t0].[CategoryID]) AS [t1] ON [t.Products].[CategoryID] = [t1].[CategoryID]ORDER BY [t1].[CategoryID]

exec sp_executesql N'SET NOCOUNT ON;DELETE FROM [Product]WHERE [ProductID] = @p0;SELECT @@ROWCOUNT;',N'@p0 int',@p0=2

exec sp_executesql N'SET NOCOUNT ON;DELETE FROM [Product]WHERE [ProductID] = @p0;SELECT @@ROWCOUNT;',N'@p0 int',@p0=3

3. 级联删除

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