不支持嵌套查询。Operation1='UnionAll‘Operation2='MultiStreamNest’

Question

我有一个以下形式的Linq to Entities查询：

var x = from a in SomeData
    where ... some conditions ...
    select new MyType
    {
        Property = a.Property,
        ChildCollection = from b in a.Children
                        select new MyChildType
                        {
                            SomeProperty = b.Property,
                            AnotherProperty = b.AnotherProperty
                        }
    };

var y = from a in SomeData
    where ... some other conditions ...
    select new MyType
    {
        Property = a.Property,
        ChildCollection = from b in a.Children
                        select new MyChildType
                        {
                            SomeProperty = b.Property,
                            AnotherProperty = b.AnotherProperty
                        }
    };

var results = x.Concat(y);

(这是一个简化的例子- 'where‘和'select’子句比这里显示的复杂。我使用单独的查询语句，因为创建一个组合的语句太复杂了，有太多的条件，并且需要很长时间才能编译)

编译正常，但在执行时失败，并出现异常：

"The nested query is not supported. Operation1='UnionAll' Operation2='MultiStreamNest'

注意，我正在尝试投射到一个嵌套的类型化结构中。如果我在Concat()之前调用x和y上的.ToList()，它工作得很好。另外，我的一个属性是枚举，但我使用整数包装器属性将其赋值给它。

有没有一种方法，我可以做我想做的事情，而不必把所有的数据都放到内存中？或者是枚举导致了失败？

谢谢,

T

Answer 1

您是否尝试过

 var results = x.Union(y);

Tiz

或

var x = (from a in SomeData
where ... some conditions ...
select new MyType
{
    Property = a.Property,
    ChildCollection = (from b in a.Children
                    select new MyChildType
                    {
                        SomeProperty = b.Property,
                        AnotherProperty = b.AnotherProperty
                    }).ToArray() //or DefaultIfEmpty
}).Concat(
from a in SomeData
where ... some other conditions ...
select new MyType
{
    Property = a.Property,
    ChildCollection = (from b in a.Children
                    select new MyChildType
                    {
                        SomeProperty = b.Property,
                        AnotherProperty = b.AnotherProperty
                    }).ToArray() //or DefaultIfEmpty
});

Answer 2

我在尝试将多组导航属性连接或联合到单个IEnumerable时遇到了类似的问题，以下是代码示例：

var requiredDocuments =                
                 (from x in db.RequestTypes where (some condition) select x.RequiredDocuments)
                 .SelectMany(r => r).ToList<DataModel.RequiredDocument>()
                 .Concat(
                 (from c in db.Categories where (some condition) select c.RequiredDocuments)
                 .SelectMany(r => r).ToList<DataModel.RequiredDocument>()
                 )
                 .Concat(
                 (from f in db.Fields where (some condition) select f.RequiredDocuments)
                 .SelectMany(r => r).ToList<DataModel.RequiredDocument>()
                );

Answer 3

如果我正确地理解了您要做的事情，我已经多次遇到相同的问题。底线是，不支持使用嵌套投影进行联合，如果需要这样做，则必须首先使用ToList实现结果。