Wcf 服务查询中的内部查询抛出“NotSupportedException”

Question

我有一个具有多对多关系的类，并且在正确过滤使用 Wcf 数据服务（通过 DataServiceQuery 类）进行的查询的结果时遇到问题。该服务公开了一个 Entity Framework 5 模型。

例如这个简单的查询：

from device in devices
select new 
{
    DeviceName = device.Name,
    TestUsers = device
        .Allocations
        .Where(allocation => allocation.User == "testUser")
}

这给了我一个NotSupportedException，在运行时：

[NotSupportedException: Constructing or initializing instances of the type <>f__AnonymousType0`4[System.String,System.String,System.String,System.Collections.Generic.IEnumerable`1[Mobiltec.M3S.Model.AllocInfo]] with the expression device.Allocations.Where(_allocation => (_allocation.User == "testUser")) is not supported.]
   System.Data.Services.Client.NonEntityProjectionAnalyzer.VisitMethodCall(MethodCallExpression m) +650
   System.Data.Services.Client.ALinqExpressionVisitor.Visit(Expression exp) +456
   System.Data.Services.Client.ALinqExpressionVisitor.VisitExpressionList(ReadOnlyCollection`1 original) +107
   System.Data.Services.Client.ALinqExpressionVisitor.VisitNew(NewExpression nex) +52
   System.Data.Services.Client.NonEntityProjectionAnalyzer.VisitNew(NewExpression nex) +226
   System.Data.Services.Client.ALinqExpressionVisitor.Visit(Expression exp) +552
   System.Data.Services.Client.NonEntityProjectionAnalyzer.Analyze(Expression e, PathBox pb, DataServiceContext context) +285
   System.Data.Services.Client.ProjectionAnalyzer.Analyze(LambdaExpression e, PathBox pb, DataServiceContext context) +226
   System.Data.Services.Client.ProjectionAnalyzer.AnalyzeResourceExpression(LambdaExpression lambda, ResourceExpression resource, DataServiceContext context) +58
   System.Data.Services.Client.ProjectionAnalyzer.Analyze(LambdaExpression le, ResourceExpression re, Boolean matchMembers, DataServiceContext context) +335
   System.Data.Services.Client.ResourceBinder.AnalyzeProjection(MethodCallExpression mce, SequenceMethod sequenceMethod, Expression& e) +1000
   System.Data.Services.Client.ResourceBinder.VisitMethodCall(MethodCallExpression mce) +149
   System.Data.Services.Client.ALinqExpressionVisitor.Visit(Expression exp) +456
   System.Data.Services.Client.ALinqExpressionVisitor.VisitExpressionList(ReadOnlyCollection`1 original) +107
   System.Data.Services.Client.ALinqExpressionVisitor.VisitMethodCall(MethodCallExpression m) +87
   System.Data.Services.Client.ResourceBinder.VisitMethodCall(MethodCallExpression mce) +177
   System.Data.Services.Client.ALinqExpressionVisitor.Visit(Expression exp) +456
   System.Data.Services.Client.ResourceBinder.Bind(Expression e, DataServiceContext context) +57
   System.Data.Services.Client.DataServiceQueryProvider.Translate(Expression e) +252
   System.Data.Services.Client.DataServiceQuery`1.Translate() +37
   System.Data.Services.Client.DataServiceRequest.GetQuerySetCount(DataServiceContext context) +77
   System.Data.Services.Client.DataServiceQueryProvider.ReturnSingleton(Expression expression) +332

如果我只在那里投射数据，而不是过滤，像这样：

from device in devices
select new 
{
    DeviceName = device.Name,
    AllocatedUsers = device
        .Allocations
        .Select(allocation => allocation.User)
}

它按预期工作。

Answer 1

OData 的 $select 子句（Linq select 在 URL 中映射到的内容）不支持转换属性值的投影，至少在 OData v3 中是这样。投影更多地用于减少您需要使用的属性数量，而不是用于操作属性值。

所以这很好并且支持：

from device in devices
select new 
{
    DeviceName = device.Name,
    TestUsers = device.Allocations
}

但是，一旦您将 .Where 子句放在 Allocations 上，就无法将其转换为 OData URL 语法。

Answer 2

尝试测试是否相等而不是分配

.Where(allocation => allocation.User == "testUser")

注意双重 ==

Answer 3

实际上你是在 where 语句中做赋值

from device in devices
select new 
{
    DeviceName = device.Name,
    TestUser = device
        .Allocations
        .Where(allocation => allocation.User = "testUser") // use == for equality
}