身份用戶管理器DeleteAsync DbUpdateConcurrencyException

Question

我試圖通過webapi后面的aspnetcore.identity UserManager刪除用戶。

    [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);
        }

    }

這將引發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)

我知道這個例外通常表示比賽條件，但我不知道為什么會這樣。

難道我做錯了什么？

編輯

我發布的用戶對象看起來像這樣：

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

Answer 1

實體框架核心使用樂觀並發 ：

在樂觀並發模型中，如果在用戶從數據庫中接收到值之后，另一個用戶在第一個用戶嘗試修改值之前修改了該值，則認為發生了違規。

將此與悲觀並發進行對比：

在悲觀的並發模型中，更新行的用戶將建立鎖。 在用戶完成更新並釋放鎖之前，沒有其他人可以更改該行。

為了實現樂觀並發， IdentityUser類包含一個ConcurrencyStamp屬性（以及數據庫中的相應列），它是GUID的字符串表示形式：

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

每次將用戶保存到數據庫時， ConcurrencyStamp都會設置為新的GUID。

以刪除用戶為例，發送到服務器的DELETE SQL語句的簡化版本可能類似於以下內容：

DELETE FROM dbo.AspNetUsers WHERE Id = '<USER_ID>' AND ConcurrencyStamp = '<CONCURRENCY_STAMP>'

當上面的SQL語句中的CONCURRENCY_STAMP值與給定用戶的數據庫中存儲的值不匹配時，會出現您收到的錯誤消息。 這樣可以確保，如果您從數據庫（包含特定的ConcurrencyStamp ）中檢索用戶，則只有在其他地方沒有其他更改的情況下，才能將更改保存到數據庫中（因為您要提供數據庫中存在的相同ConcurrencyStamp值） ）。

從上面的ConcurrencyStamp定義可以看到，該類屬性默認為新的GUID每次創建IdentityUser （或子類）時，都會為其分配新的ConcurrencyStamp值。 在您的示例中，通過將User傳遞給DeleteAsync操作，ASP.NET Core Model-Binding首先創建一個User的新實例，然后設置JSON有效負載中存在的屬性。 由於有效負載中沒有ConcurrencyStamp值，因此User最終將獲得一個新的 ConcurrencyStamp值，該值當然與數據庫中的值不匹配。

為避免此問題，您可以將ConcurrencyStamp值添加到客戶端發送的有效負載中。 但是，我不建議這樣做。 解決此問題的最簡單，最安全的方法是，簡單地發送User的Id作為有效載荷，使用_userManager.FindByIdAsync檢索User本身，然后使用該實例執行刪除。 這是一個例子：

[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);
    }
}