连续获取最大值时如何解决错误（EF Core Linq）？ [复制]

Question

访问具有值的表： Id ImageId Score 。 需要得到每个模拟ImageId的最高分。

例如：

1 a1 10 points
2 a1 30 points
3 b1 15 points

想要：a1 30 points and b1 15 points

使用的代码：

public async Task<List<HistoryEntry>> Handle(Request request, CancellationToken cancellationToken)
                => await _db.History.GroupBy(a => a.ImageId).Select(s => s.OrderByDescending(x => x.Score).First()).ToListAsync();
        }

错误：

.OrderByDescending(x => x.Score)' 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 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or 'ToListAsync'. See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.

InvalidOperationException: The LINQ expression 'GroupByShaperExpression: KeySelector: h.ImageId, ElementSelector:EntityShaperExpression: EntityType: HistoryEntry ValueBufferExpression: ProjectionBindingExpression: EmptyProjectionMember IsNullable: False .OrderByDescending(x => x.Score)' 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 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or 'ToListAsync'. See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.

Answer 1

代码中最简单的方法是更改​​顺序：

var groupedResult  = await _db.History.GroupBy(a => a.ImageId).ToListAsync();
return groupedResult.Select(s => s.OrderByDescending(x => x.Score).First());

EF 无法将此类嵌套查询转换为 SQL - 因此您的错误。在此解决方案中，分组结果在内存中具体化，因此此类嵌套查询不会导致问题。 不过要小心 - 如果数据库中有很多记录，这肯定不是最佳解决方案，但在记录很少的情况下，它会发挥作用。