linq - Web API、OData、EF5、联合 : The 'Distinct' operation cannot be applied to the collection ResultType of the specified argument

Question

我正在使用 OData Nuget 包开发 Web API 原型(prototype)。
我在使 LINQ to EF 查询正常工作时遇到了一些问题。

Here are my data model.它已被高度简化。

我正在尝试使用此 DTO 使查询正常工作:

public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
    public IEnumerable<Order> Orders { get; set; }
}

public class Order
{
    public int Id { get; set; }
    public DateTime Date { get; set; }
}

查询如下所示:

[Queryable]
public IQueryable<Product> Get()
{
    var productA = _context.ProductA
                            .Select(p => new Product
                                {
                                    Id = p.id,
                                    Name = p.name,
                                    Orders = p.ProductAOrders.Select(o => new Order
                                        {
                                            Id = o.OrderId,
                                            Date = o.Orders.Date,
                                        })
                                });

    var productB = _context.ProductB
                            .Select(p => new Product
                                {
                                    Id = p.Id,
                                    Name = p.Name,
                                    Orders = p.ProductBOrders.Select(o => new Order
                                        {
                                            Id = o.OrderId,
                                            Date = o.Orders.Date,
                                        })
                                });

    return productA.Union(productB);
}

当尝试联合这两个查询时，我收到此错误:

<Error><Message>An error has occurred.</Message><ExceptionMessage>The 'Distinct' operation cannot be applied to the collection ResultType of the specified argument.
Parameter name: argument</ExceptionMessage><ExceptionType>System.ArgumentException</ExceptionType><StackTrace>   at System.Data.Common.CommandTrees.ExpressionBuilder.Internal.ArgumentValidation.ValidateDistinct(DbExpression argument)
   at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.UnionTranslator.TranslateBinary(ExpressionConverter parent, DbExpression left, DbExpression right)
   at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.BinarySequenceMethodTranslator.Translate(ExpressionConverter parent, MethodCallExpression call)
   at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.SequenceMethodTranslator.Translate(ExpressionConverter parent, MethodCallExpression call, SequenceMethod sequenceMethod)
   at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.TypedTranslate(ExpressionConverter parent, MethodCallExpression linq)
   at System.Data.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)
   at System.Data.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
   at System.Data.Objects.ELinq.ExpressionConverter.Convert()
   at System.Data.Objects.ELinq.ELinqQueryState.GetExecutionPlan(Nullable`1 forMergeOption)
   at System.Data.Objects.ObjectQuery.ToTraceString()
   at System.Data.Entity.Internal.Linq.InternalQuery`1.ToString()
   at System.Data.Entity.Infrastructure.DbQuery`1.ToString()
   at System.Convert.ToString(Object value, IFormatProvider provider)
   at System.Web.Http.Tracing.Tracers.HttpActionDescriptorTracer.&lt;ExecuteAsync&gt;b__2(TraceRecord tr, Object value)
   at System.Web.Http.Tracing.ITraceWriterExtensions.&lt;&gt;c__DisplayClass1b`1.&lt;&gt;c__DisplayClass1f.&lt;TraceBeginEndAsync&gt;b__13(TraceRecord traceRecord)
   at System.Web.Http.Tracing.SystemDiagnosticsTraceWriter.Trace(HttpRequestMessage request, String category, TraceLevel level, Action`1 traceAction)
   at System.Web.Http.Tracing.ITraceWriterExtensions.&lt;&gt;c__DisplayClass1b`1.&lt;TraceBeginEndAsync&gt;b__12(TResult result)
   at System.Threading.Tasks.TaskHelpersExtensions.&lt;&gt;c__DisplayClass3b`2.&lt;Then&gt;b__3a(Task`1 t)
   at System.Threading.Tasks.TaskHelpersExtensions.ThenImpl[TTask,TOuterResult](TTask task, Func`2 continuation, CancellationToken cancellationToken, Boolean runSynchronously)
</StackTrace></Error>

我可以返回 productA 或 productB - 但返回这 2 个查询的 Union 会导致上述明显错误。

对我可能做错的事情有任何想法吗？

Answer 1

看起来像一个EF错误。我假设您正在尝试使 MEST(相同类型的多个实体集)工作。您可以尝试，而不是您的查询，

public IQueryable<Product> Get()
{
    var productA = _context.ProductA
                            .Select(p => new Product
                            {
                                Id = p.id,
                                Name = p.name,
                            });

    var productB = _context.ProductB
                            .Select(p => new Product
                            {
                                Id = p.Id,
                                Name = p.Name,
                            });

    return
        productA
        .Union(productB)
        .Select(p => new Product
        {
            Id = p.Id,
            Name = p.Name,
            Orders = _context.Orders
                    .Where(o => o.ProductA.Id == p.Id || o.ProductB.Id == p.Id)
                    .Select(o => new Order
                    {
                        Id = o.OrderId,
                        Date = o.Orders.Date,
                    })
        });
}

这个想法是将导航属性从联合中取出，然后再将它们添加回来。这只有在 Orders 有指向 ProductA 或 ProductB 的反向指针时才有效。