无法翻译 LINQ 表达式。要么以可以翻译的形式重写查询，要么显式切换到客户端评估

Question

我想维持child-grandchildren关系。我需要区分哪个 children 返回哪个 grandchildren.

查询returns

"InvalidOperationException: The LINQ expression 
'x =>x.Children.AsQueryable().Select(c => new 
 {
 ItemNo= c.ItemNo,
 GrandChildItemNos = c.Children.AsQueryable().Select(gc => gc.ItemNo).ToList()
 })
.ToDictionary(keySelector: c => c.ItemNo, elementSelector: c => c.GrandChildItemNos)' 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."

我认为 EF 在将 .ToDictionary() 部分翻译成 SQL 时遇到问题。谁能帮忙解决这个问题。

Answer 1

你很接近！

当您将 ToDictionary() 移动到实体化点（ToListAsync()）之外时，它将起作用。这实际上也是错误消息所说的：“或显式切换到客户端评估”。 ToDictionary 调用无法转换为查询。

var childDetails = await context.Items.Where(w => w.ItemNo == parentItemNo)
    .SelectMany(x => x.Children
        .Select(c => new
        {
            c.ItemNo,
            GrandChildItemNos = c.Children.Select(gc => gc.ItemNo)
        })
    )
    .ToListAsync();
var dictionary = childDetails.ToDictionary(d => d.ItemNo, d => d.GrandChildItemNos);

这是我的结果：

无法翻译 LINQ 表达式。要么以可以翻译的形式重写查询，要么显式切换到客户端评估

The LINQ expression could not not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly

c#

sql

entity-framework-core

asp.net-core