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• 一句话的事儿：

场景一：查询Clubs以及关联的League

//查询Clubs以及关联的League
var club = context.Clubs.Include(x => x.League).FirstOrDefault();
club.League.Name += "@";
context.SaveChanges();

场景二：考虑离线状态

//context与newContext是离线状态
var game = context.Games
    .Include(x => x.GamePlayers)
    .ThenInclude(x => x.Player)
    .FirstOrDefault();
var firstPlayer = game.GamePlayers[0].Player;
firstPlayer.Name += "$";
{
    using var newContext = new DataContext();
    newContext.Players.Update(firstPlayer);
    newContext.SaveChanges();
}

减少Update次数

newContext.Entry(firstPlayer).State = EntityState.Modified;

场景三：如何设置多对多的关系

通过已知的Game与Player创建GamePlayer

var gamePlayer = new GamePlayer
{
    GameId = 1,
    PlayerId = 5
};
context.Add(gamePlayer);
context.SaveChanges();

game通过查询，player通过命名

var game = context.Games.First();
game.GamePlayers.Add(new GamePlayer { PlayerId = 4 });
context.SaveChanges();

场景四：删除多对多关系->删除GamePlayer

//先将GamePlayer查询出来
//也可以new一个出来
var gamePlayer = new GamePlayer { GameId = 1, PlayerId = 5 };
context.Remove(gamePlayer);
context.SaveChanges();

场景五：更新多对多的关系

//01.首先解除二者之间的关系，然后重新创建关系

场景六：修改一对一的关系

//修改一对一的关系
var player = context.Players.First();
player.Resume = new Resume { Description = "Update Description!!!" };
context.SaveChanges();

主键为空：为唯一索引

场景七：修改一对一（离线状态）

var player = context.Players
    .AsNoTracking()
    .OrderBy(x => x.Id)
    .Last();
player.Resume = new Resume { Description = "update description in offline." };
{
    using var newContext = new DataContext();
    newContext.Attach(player);
    newContext.SaveChanges();
}

接下来我们做一个事情，上面的resume我们已经添加了，那么再添加一次会发生什么呢？ :::warning Microsoft.EntityFrameworkCore.DbUpdateException
HResult=0x80131500 ::: :::warning Message=An error occurred while updating the entries. See the inner exception for details. ::: :::warning Source=Microsoft.EntityFrameworkCore.Relational ::: :::warning StackTrace: ::: :::warning at Microsoft.EntityFrameworkCore.Update.ReaderModificationCommandBatch.Execute(IRelationalConnection connection) ::: :::warning at Microsoft.EntityFrameworkCore.Update.Internal.BatchExecutor.Execute(IEnumerable1 commandBatches, IRelationalConnection connection) ::: :::warning at Microsoft.EntityFrameworkCore.Storage.RelationalDatabase.SaveChanges(IList1 entries) ::: :::warning at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.SaveChanges(IList1 entriesToSave) ::: :::warning at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.SaveChanges(DbContext _, Boolean acceptAllChangesOnSuccess) ::: :::warning at Microsoft.EntityFrameworkCore.SqlServer.Storage.Internal.SqlServerExecutionStrategy.Execute[TState,TResult](TState state, Func3 operation, Func`3 verifySucceeded) ::: :::warning at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.SaveChanges(Boolean acceptAllChangesOnSuccess) ::: :::warning at Microsoft.EntityFrameworkCore.DbContext.SaveChanges(Boolean acceptAllChangesOnSuccess) ::: :::warning at Microsoft.EntityFrameworkCore.DbContext.SaveChanges() ::: :::warning at Demo.App.Program.Main(String[] args) in D:\Projects\TutorialOnline\Entity Framework Core 3.1 极简入门教程\EFCoreDemo3.x\Demo.App\Program.cs:line 301 ::: :::warning This exception was originally thrown at this call stack: ::: :::warning [External Code] ::: :::warning Inner Exception 1: ::: :::warning SqlException: Cannot insert duplicate key row in object ‘dbo.Resumes’ with unique index ‘IX_Resumes_PlayerId’. The duplicate key value is (7). ::: :::warning The statement has been terminated. ::: 这是因为PlayerId是唯一索引导致的

var player = context.Players
     .Include(x => x.Resume)
     .OrderBy(x => x.Id)
     .Last();
player.Resume = new Resume { Description = "update description in offline." };
context.SaveChanges();

• 本文作者：GeekPower - Felix Sun
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