Linq to SQL 无法翻译，因为不支持 RowNumberExpression

Question

我一直在尝试运行查询，我想按特定属性对所有记录进行分组，然后 select 每个列表中的第一个。我已经经历了几次这样的迭代，但可以肯定地说，EF Core 3.0 及更高版本使得这很难 activity 实现。我想我可能会遇到 JET 提供程序的限制，但是当我收到有关 RowNumberExpression 无法翻译的错误时。有问题的数据库是一个很旧的 Access 97 数据库，我知道它很旧，不应该使用，但我在这里没有选择。

总之我暂时得到的查询如下：

var sons = await _snContext.TDespatch
                .Select(x => x.OrderNumber)
                .Distinct()
                .SelectMany(x => _snContext.TDespatch.Where(d => x == d.OrderNumber).Take(1))
                .ToArrayAsync();

我已经使用 GroupBy 进行了几次其他迭代，然后从每个组中取第一个，但它产生了相似的结果。

我得到的错误如下：

Microsoft.EntityFrameworkCore.Query: Error: An exception occurred while iterating over the results of a query for context type 'SNDBConnector.Contexts.SNDBContext'.
System.InvalidOperationException: The LINQ expression '[Microsoft.EntityFrameworkCore.Query.SqlExpressions.RowNumberExpression]' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to either AsEnumerable(), AsAsyncEnumerable(), ToList(), or ToListAsync(). See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.
   at EntityFrameworkCore.Jet.Query.Sql.Internal.JetQuerySqlGenerator.VisitRowNumber(RowNumberExpression rowNumberExpression)
   at Microsoft.EntityFrameworkCore.Query.SqlExpressionVisitor.VisitExtension(Expression extensionExpression)
   at System.Linq.Expressions.Expression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at Microsoft.EntityFrameworkCore.Query.QuerySqlGenerator.VisitProjection(ProjectionExpression projectionExpression)
   at Microsoft.EntityFrameworkCore.Query.SqlExpressionVisitor.VisitExtension(Expression extensionExpression)
   at System.Linq.Expressions.Expression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at EntityFrameworkCore.Jet.Query.Sql.Internal.JetQuerySqlGenerator.<VisitSelect>b__7_0(ProjectionExpression e)
   at EntityFrameworkCore.Jet.Query.Sql.Internal.JetQuerySqlGenerator.GenerateList[T](IReadOnlyList`1 items, Action`1 generationAction, Action`1 joinAction)
   at EntityFrameworkCore.Jet.Query.Sql.Internal.JetQuerySqlGenerator.VisitSelect(SelectExpression selectExpression)
   at Microsoft.EntityFrameworkCore.Query.SqlExpressionVisitor.VisitExtension(Expression extensionExpression)
   at System.Linq.Expressions.Expression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at EntityFrameworkCore.Jet.Query.Sql.Internal.JetQuerySqlGenerator.VisitSelect(SelectExpression selectExpression)
   at Microsoft.EntityFrameworkCore.Query.SqlExpressionVisitor.VisitExtension(Expression extensionExpression)
   at System.Linq.Expressions.Expression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at Microsoft.EntityFrameworkCore.Query.QuerySqlGenerator.VisitInnerJoin(InnerJoinExpression innerJoinExpression)
   at Microsoft.EntityFrameworkCore.Query.SqlExpressionVisitor.VisitExtension(Expression extensionExpression)
   at System.Linq.Expressions.Expression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at EntityFrameworkCore.Jet.Query.Sql.Internal.JetQuerySqlGenerator.VisitSelect(SelectExpression selectExpression)
   at Microsoft.EntityFrameworkCore.Query.QuerySqlGenerator.GetCommand(SelectExpression selectExpression)
   at Microsoft.EntityFrameworkCore.Query.Internal.RelationalCommandCache.GetRelationalCommand(IReadOnlyDictionary`2 parameters)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable`1.AsyncEnumerator.InitializeReaderAsync(DbContext _, Boolean result, CancellationToken cancellationToken)
   at EntityFrameworkCore.Jet.Storage.Internal.JetExecutionStrategy.ExecuteAsync[TState,TResult](TState state, Func`4 operation, Func`4 verifySucceeded, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable`1.AsyncEnumerator.MoveNextAsync()

System.InvalidOperationException: The LINQ expression '[Microsoft.EntityFrameworkCore.Query.SqlExpressions.RowNumberExpression]' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to either AsEnumerable(), AsAsyncEnumerable(), ToList(), or ToListAsync(). See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.
   at EntityFrameworkCore.Jet.Query.Sql.Internal.JetQuerySqlGenerator.VisitRowNumber(RowNumberExpression rowNumberExpression)
   at Microsoft.EntityFrameworkCore.Query.SqlExpressionVisitor.VisitExtension(Expression extensionExpression)
   at System.Linq.Expressions.Expression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at Microsoft.EntityFrameworkCore.Query.QuerySqlGenerator.VisitProjection(ProjectionExpression projectionExpression)
   at Microsoft.EntityFrameworkCore.Query.SqlExpressionVisitor.VisitExtension(Expression extensionExpression)
   at System.Linq.Expressions.Expression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at EntityFrameworkCore.Jet.Query.Sql.Internal.JetQuerySqlGenerator.<VisitSelect>b__7_0(ProjectionExpression e)
   at EntityFrameworkCore.Jet.Query.Sql.Internal.JetQuerySqlGenerator.GenerateList[T](IReadOnlyList`1 items, Action`1 generationAction, Action`1 joinAction)
   at EntityFrameworkCore.Jet.Query.Sql.Internal.JetQuerySqlGenerator.VisitSelect(SelectExpression selectExpression)
   at Microsoft.EntityFrameworkCore.Query.SqlExpressionVisitor.VisitExtension(Expression extensionExpression)
   at System.Linq.Expressions.Expression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at EntityFrameworkCore.Jet.Query.Sql.Internal.JetQuerySqlGenerator.VisitSelect(SelectExpression selectExpression)
   at Microsoft.EntityFrameworkCore.Query.SqlExpressionVisitor.VisitExtension(Expression extensionExpression)
   at System.Linq.Expressions.Expression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at Microsoft.EntityFrameworkCore.Query.QuerySqlGenerator.VisitInnerJoin(InnerJoinExpression innerJoinExpression)
   at Microsoft.EntityFrameworkCore.Query.SqlExpressionVisitor.VisitExtension(Expression extensionExpression)
   at System.Linq.Expressions.Expression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at EntityFrameworkCore.Jet.Query.Sql.Internal.JetQuerySqlGenerator.VisitSelect(SelectExpression selectExpression)
   at Microsoft.EntityFrameworkCore.Query.QuerySqlGenerator.GetCommand(SelectExpression selectExpression)
   at Microsoft.EntityFrameworkCore.Query.Internal.RelationalCommandCache.GetRelationalCommand(IReadOnlyDictionary`2 parameters)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable`1.AsyncEnumerator.InitializeReaderAsync(DbContext _, Boolean result, CancellationToken cancellationToken)
   at EntityFrameworkCore.Jet.Storage.Internal.JetExecutionStrategy.ExecuteAsync[TState,TResult](TState state, Func`4 operation, Func`4 verifySucceeded, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable`1.AsyncEnumerator.MoveNextAsync()
Exception thrown: 'System.InvalidOperationException' in System.Private.CoreLib.dll

我在堆栈溢出时看到了此问题的其他解决方案，但其中 none 似乎完全没有帮助。这对我来说似乎是一件简单的事情，但它也似乎是一项不可能完成的任务，但当我认为这是一项简单的任务时，也许我被蒙蔽了。

如有任何帮助，我们将不胜感激。

Answer 1

这是 EF 核心 6 的已知限制。

除非您的数据库具有允许 OrderNumber->FirstDespatch 导航的关系，否则您需要进行子查询。此外，您应该添加一个 order by 子句以使结果一致。

private async IAsyncEnumerable<TDespatch> GetFirstByOrderNumberAsync()
{
  foreach(var number in await _snContext.TDespatch
      .Select(x => x.OrderNumber)
      .Distinct()
      .ToListAsync()
      .ConfigureAwait(false))
  {
    yield return await _snContext.TDespatch
      .AsNoTracking()
      .Where(despatch => number == despatch.OrderNumber)
      // insert an order by clause here
      .FirstAsync()
      .ConfigureAwait(false);
  }
}

Answer 2

因此，我与团队中的一名 DBA 合作，我们设法提出了我需要的 SQL 版本，当以原始 SQL 形式直接注入 EF 时，它的效果非常好。我采取的下一步是下载一个名为 Linqer 的程序。我创建了一个 SQL 服务器版本的 Access 数据库，以便 Linqer 可以连接到它并进行测试，然后我将查询的 SQL 版本放入 Linqer，它会自动将 SQL 转换为林克。通过一些调整，它给了我我想要的东西。如果有人感兴趣，Linq 在这里：

await (from Despatch in context.Despatch
       where Despatch.DespatchDate >= startDate && Despatch.DespatchDate <= endDate
       group Despatch by new
       {
           Despatch.OrderNumber
       } into g
       select new Despatch
       {
           OrderNumber = g.Key.OrderNumber,
           DespatchDate = g.Min(p => p.DespatchDate),
           RepCalNumber = g.Min(p => p.RepCalNumber),
           SerialNumber = g.Min(p => p.SerialNumber)
       })
       .OrderByDescending(x => x.DespatchDate)
       .ToArrayAsync();

这在那个巨大的 Access 数据库上运行得非常快，尤其是在我添加了日期限制的情况下。我认为这里的关键是使用 Access 能够理解的 Min 函数。感谢那些试图帮助我的人！

Linq to SQL 无法翻译，因为不支持 RowNumberExpression

Linq to SQL cannot translate because RowNumberExpression isn't supported

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