身份用户管理器DeleteAsync DbUpdateConcurrencyException

Question

我试图通过webapi后面的aspnetcore.identity UserManager删除用户。

    [HttpPost("Delete", Name = "DeleteRoute")]
    [Authorize(Roles = "SuperUser")]
    public async Task<IActionResult> DeleteAsync([FromBody] User user)
    {
        Console.WriteLine("Deleting user: " + user.Id);
        try {
            await _userManager.DeleteAsync(user);
            return Ok();
        } catch(Exception e) {
            return BadRequest(e.Message);
        }

    }

这引发了 DbUpdateConcurrencyException

   Microsoft.EntityFrameworkCore.DbUpdateConcurrencyException: Database operation expected to affect 1 row(s) but actually affected 0 row(s). Data may have been modified or deleted since entities were loaded. See http://go.microsoft.com/fwlink/?LinkId=527962 for information on understanding and handling optimistic concurrency exceptions.
   at Microsoft.EntityFrameworkCore.Update.AffectedCountModificationCommandBatch.ThrowAggregateUpdateConcurrencyException(Int32 commandIndex, Int32 expectedRowsAffected, Int32 rowsAffected)
   at Microsoft.EntityFrameworkCore.Update.AffectedCountModificationCommandBatch.ConsumeResultSetWithoutPropagationAsync(Int32 commandIndex, RelationalDataReader reader, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Update.AffectedCountModificationCommandBatch.ConsumeAsync(RelationalDataReader reader, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Update.ReaderModificationCommandBatch.ExecuteAsync(IRelationalConnection connection, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Update.Internal.BatchExecutor.ExecuteAsync(DbContext _, ValueTuple`2 parameters, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.SqlServer.Storage.Internal.SqlServerExecutionStrategy.ExecuteAsync[TState,TResult](TState state, Func`4 operation, Func`4 verifySucceeded, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.SaveChangesAsync(IReadOnlyList`1 entriesToSave, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.SaveChangesAsync(Boolean acceptAllChangesOnSuccess, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.DbContext.SaveChangesAsync(Boolean acceptAllChangesOnSuccess, CancellationToken cancellationToken)
Microsoft.EntityFrameworkCore.DbUpdateConcurrencyException: Database operation expected to affect 1 row(s) but actually affected 0 row(s). Data may have been modified or deleted since entities were loaded. See http://go.microsoft.com/fwlink/?LinkId=527962 for information on understanding and handling optimistic concurrency exceptions.
   at Microsoft.EntityFrameworkCore.Update.AffectedCountModificationCommandBatch.ThrowAggregateUpdateConcurrencyException(Int32 commandIndex, Int32 expectedRowsAffected, Int32 rowsAffected)
   at Microsoft.EntityFrameworkCore.Update.AffectedCountModificationCommandBatch.ConsumeResultSetWithoutPropagationAsync(Int32 commandIndex, RelationalDataReader reader, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Update.AffectedCountModificationCommandBatch.ConsumeAsync(RelationalDataReader reader, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Update.ReaderModificationCommandBatch.ExecuteAsync(IRelationalConnection connection, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Update.Internal.BatchExecutor.ExecuteAsync(DbContext _, ValueTuple`2 parameters, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.SqlServer.Storage.Internal.SqlServerExecutionStrategy.ExecuteAsync[TState,TResult](TState state, Func`4 operation, Func`4 verifySucceeded, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.SaveChangesAsync(IReadOnlyList`1 entriesToSave, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.SaveChangesAsync(Boolean acceptAllChangesOnSuccess, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.DbContext.SaveChangesAsync(Boolean acceptAllChangesOnSuccess, CancellationToken cancellationToken)

我知道这个例外通常表示比赛条件，但我不知道为什么会这样。

难道我做错了什么？

编辑

我发布的用户对象看起来像这样：

"User": {
 "Email": "",
 "FirstName": "",
 "LastName": "",
 "Gender": "",
 "Affiliation": {
     "isStudent": true,
     "isEmployee": false
   }
   ...
}

Answer 1

实体框架核心使用乐观并发：

在乐观并发模型中，如果在用户从数据库中接收到值之后，另一个用户在第一个用户尝试修改它之前修改了该值，则认为发生了违规。

将此与悲观并发进行对比：

在悲观的并发模型中，更新行的用户将建立锁。在用户完成更新并释放锁之前，没有其他人可以更改该行。

为了实现乐观并发，IdentityUser该类包含一个ConcurrencyStamp属性（以及数据库中的相应列），该属性是GUID的字符串表示形式：

public virtual string ConcurrencyStamp { get; set; } = Guid.NewGuid().ToString();

每次将用户保存到数据库时，ConcurrencyStamp都会将其设置为新的GUID。

以删除用户为例，DELETE发送到服务器的SQL语句的简化版本可能类似于以下内容：

public virtual string ConcurrencyStamp { get; set; } = Guid.NewGuid().ToString();

当CONCURRENCY_STAMP上面的SQL语句中的值与给定用户的数据库中存储的值不匹配时，会出现您收到的错误消息。这样可以确保，如果您从数据库（包含特定的ConcurrencyStamp）中检索用户，则只有在其他地方未进行其他更改（因为您提供ConcurrencyStamp的数据库中存在的值相同）时，才可以将更改保存到数据库中。

从ConcurrencyStamp上面的定义可以看到，该属性默认为新属性GUID-每次IdentityUser创建（或子类）时，它都会获得一个新ConcurrencyStamp值。在您的示例中，通过将User传递给您的DeleteAsync操作，ASP.NET Core Model-Binding首先创建的新实例，User然后设置JSON有效负载中存在的属性。由于ConcurrencyStamp有效负载中没有值，User因此最终将得到一个新 ConcurrencyStamp值，该值与数据库中的值不匹配。

为避免此问题，可以将ConcurrencyStamp值添加到客户端发送的有效负载中。但是，我不建议这样做。解决这个问题的最简单，最安全的办法是发送Id的User作为有效载荷，检索User自己使用_userManager.FindByIdAsync，然后使用该实例来执行删除操作。这是一个例子：

[HttpPost("Delete/{id}", Name = "DeleteRoute")]
[Authorize(Roles = "SuperUser")]
public async Task<IActionResult> DeleteAsync(string id)
{
    Console.WriteLine("Deleting user: " + id);
    try {
        var user = await _userManager.FindByIdAsync(id);

        if(user == null)
            // ... 

        await _userManager.DeleteAsync(user);
        return Ok();
    } catch(Exception e) {
        return BadRequest(e.Message);
    }
}