无法翻译给定的 'GroupBy' 模式。在 'GroupBy' 之前调用 'AsEnumerable' 以在客户端对其进行评估

Question

我有一个项目列表“listItems”，就像这样

Id Code Value
1  'a'  '1'
2  'a'  '2'
3  'b'  'x'
4  'b'  'y'

并获得以下

'a' => '1', '2'
'b' => 'x', 'y'

我通过 EF Core 使用以下代码

public async Task<Dictionary<string, List<string>>> GetAllListsAsync() =>
    await _context.ListItems
            .GroupBy(x => x.Code)
            .OrderBy(x => x.Key)
            .ToDictionaryAsync(x => x.Key, x => x.Select(y => y.Value)
                                                 .OrderBy(v => v)
                                                 .ToList());

我在Swagger的API中得到以下错误：

System.InvalidOperationException: Unable to translate the given 'GroupBy' pattern. Call 'AsEnumerable' before 'GroupBy' to evaluate it client-side. at Microsoft.EntityFrameworkCore.Query.RelationalShapedQueryCompilingExpressionVisitor.ShaperProcessingExpressionVisitor.VisitExtension(Expression extensionExpression) at Microsoft.EntityFrameworkCore.Query.RelationalShapedQueryCompilingExpressionVisitor.ShaperProcessingExpressionVisitor.ProcessShaper(Expression shaperExpression, RelationalCommandCache& relationalCommandCache, LambdaExpression& relatedDataLoaders) at Microsoft.EntityFrameworkCore.Query.RelationalShapedQueryCompilingExpressionVisitor.VisitShapedQuery(ShapedQueryExpression shapedQueryExpression)

---

我尝试做的事情：

• 如果我在 GroupBy 之前使用 AsEnumerable，它将不再找到 ToDictionaryAsync：

• 我测试了在AsEnumlerable之后添加AsQueryable编译错误信息在编译时消失了，但在运行时保持不变（无法翻译'GroupBy'模式。在...之前调用 'AsEnumerable')

• 这个我也试过了

    public async Task<Dictionary<string, List<string>>> GetAllListsAsync() =>
        await _context.ListItems
            .ToLookup(x => x.Code, x => x.Value)
            .AsQueryable()
            .ToDictionaryAsync(x => x.Key, x => x.Select(y => y)
                                                 .ToList());
  

运行时在那种情况下说：

The source 'IQueryable' doesn't implement 'IAsyncEnumerable<System.Linq.IGrouping2[System.String,System.String]>'. Only sources that implement 'IAsyncEnumerable' can be used for Entity Framework asynchronous operations. at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.AsAsyncEnumerable[TSource](IQueryable1 source) at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.ToDictionaryAsync[TSource,TKey,TElement](IQueryable1 source, Func2 keySelector, Func2 elementSelector, IEqualityComparer1 comparer, CancellationToken cancellationToken)

Answer 1

由于加载了所有记录，因此必须在客户端进行分组。请注意，在具体化对象后，您不能使用异步可查询扩展。

public async Task<Dictionary<string, List<string>>> GetAllListsAsync() =>
   (await _context.ListItems.ToListAsync())
            .GroupBy(x => x.Code)
            .OrderBy(x => x.Key)
            .ToDictionary(x => x.Key, x => x.Select(y => y.Value)
                                                 .OrderBy(v => v)
                                                 .ToList());