entity-framework - EF Core 2.1.2 和 .net core 2.0 - Linq2Entities 导致 'must be reducible node' 错误

Question

我调查了几个小时来为我的问题找到解决方案或解决方法。我发现的只是错误“必须是可简化的节点”与 Linq Join 或旧的 EF 版本结合使用。所以我不得不再次问这个问题。

当前环境:

• VS2017
• .Net core 2.0项目
• EntityFramework 核心 2.1.2

我的这个错误的源代码是:

    // this query works...
var projectEfforts = from projectEntry in _context.ProjectTrackingEntry
                     where projectEntry.ProjectId == projectId
                     group projectEntry by new { projectEntry.Day.Year, projectEntry.Day.Month } into g
                     select new
                     {
                         PeDate = new DateTime(g.Key.Year, g.Key.Month, 1),
                         Month = g.Key,
                         MonthlyWorkingHours = g.Sum(x => x.WorkingHours)
                     };

// and here the query is executed with a valid result
string jsonprojectEfforts = Newtonsoft.Json.JsonConvert.SerializeObject(projectEfforts);

// following Linq will produce an error
var effHours = from projectEffort in projectEfforts
               where (projectEffort.PeDate < fromDate)
               select projectEffort.MonthlyWorkingHours;

// work with the result above will cause the 'must be reducible node' error
string jsoneffHours = Newtonsoft.Json.JsonConvert.SerializeObject(effHours);

为了更好地理解这里是第一个 Linq 操作的结果:

[{"PeDate":"2017-01-01T00:00:00","Month":{"Year":2017,"Month":1},"MonthlyWorkingHours":128.00} ,{"PeDate":"2018-01-01T00:00:00","Month":{"Year":2018,"Month":1},"MonthlyWorkingHours":40.00} ,{"PeDate":"2017-02-01T00:00:00","Month":{"Year":2017,"Month":2},"MonthlyWorkingHours":80.00} ,{"PeDate":"2017-03-01T00:00:00","Month":{"Year":2017,"Month":3},"MonthlyWorkingHours":88.00} ,{"PeDate":"2018-03-01T00:00:00","Month":{"Year":2018,"Month":3},"MonthlyWorkingHours":64.00} ,{"PeDate":"2017-04-01T00:00:00","Month":{"Year":2017,"Month":4},"MonthlyWorkingHours":104.00} ,{"PeDate":"2018-04-01T00:00:00","Month":{"Year":2018,"Month":4},"MonthlyWorkingHours":76.00} ,{"PeDate":"2017-05-01T00:00:00","Month":{"Year":2017,"Month":5},"MonthlyWorkingHours":16.00} ,{"PeDate":"2018-05-01T00:00:00","Month":{"Year":2018,"Month":5},"MonthlyWorkingHours":24.00} ,{"PeDate":"2017-06-01T00:00:00","Month":{"Year":2017,"Month":6},"MonthlyWorkingHours":60.00} ,{"PeDate":"2018-06-01T00:00:00","Month":{"Year":2018,"Month":6},"MonthlyWorkingHours":24.00} ,{"PeDate":"2017-07-01T00:00:00","Month":{"Year":2017,"Month":7},"MonthlyWorkingHours":48.00} ,{"PeDate":"2018-07-01T00:00:00","Month":{"Year":2018,"Month":7},"MonthlyWorkingHours":80.00} ,{"PeDate":"2017-08-01T00:00:00","Month":{"Year":2017,"Month":8},"MonthlyWorkingHours":104.00} ,{"PeDate":"2017-09-01T00:00:00","Month":{"Year":2017,"Month":9},"MonthlyWorkingHours":48.00} ,{"PeDate":"2017-10-01T00:00:00","Month":{"Year":2017,"Month":10},"MonthlyWorkingHours":72.00} ,{"PeDate":"2017-12-01T00:00:00","Month":{"Year":2017,"Month":12},"MonthlyWorkingHours":213.75}]

第二个 Linq 操作 (effHours) 的错误 发生在尝试序列化 JSON 中的结果时(上面片段中的最后一行代码):

at System.Linq.Expressions.Expression.ReduceAndCheck() at System.Linq.Expressions.Expression.ReduceExtensions() at System.Linq.Expressions.Compiler.StackSpiller.RewriteExtensionExpression(Expression expr, Stack stack) at System.Linq.Expressions.Compiler.StackSpiller.RewriteExpression(Expression node, Stack stack) at System.Linq.Expressions.Compiler.StackSpiller.RewriteMemberExpression(Expression expr, Stack stack) at System.Linq.Expressions.Compiler.StackSpiller.RewriteExpression(Expression node, Stack stack) at System.Linq.Expressions.Compiler.StackSpiller.RewriteMemberExpression(Expression expr, Stack stack) at System.Linq.Expressions.Compiler.StackSpiller.RewriteExpression(Expression node, Stack stack) at System.Linq.Expressions.Compiler.StackSpiller.ChildRewriter.Add(Expression expression) at System.Linq.Expressions.Compiler.StackSpiller.ChildRewriter.AddArguments(IArgumentProvider expressions) at System.Linq.Expressions.Compiler.StackSpiller.RewriteNewExpression(Expression expr, Stack stack) at System.Linq.Expressions.Compiler.StackSpiller.RewriteExpression(Expression node, Stack stack) at System.Linq.Expressions.Compiler.StackSpiller.ChildRewriter.Add(Expression expression) at System.Linq.Expressions.Compiler.StackSpiller.RewriteBinaryExpression(Expression expr, Stack stack) at System.Linq.Expressions.Compiler.StackSpiller.RewriteExpression(Expression node, Stack stack) at System.Linq.Expressions.Compiler.StackSpiller.RewriteExpressionFreeTemps(Expression expression, Stack stack) at System.Linq.Expressions.Compiler.StackSpiller.Rewrite[T](Expression1 lambda)
at System.Linq.Expressions.Expression1.Accept(StackSpiller spiller) at System.Linq.Expressions.Compiler.StackSpiller.RewriteLambdaExpression(Expression expr) at System.Linq.Expressions.Compiler.StackSpiller.RewriteExpression(Expression node, Stack stack) at System.Linq.Expressions.Compiler.StackSpiller.ChildRewriter.Add(Expression expression) at System.Linq.Expressions.Compiler.StackSpiller.ChildRewriter.AddArguments(IArgumentProvider expressions) at System.Linq.Expressions.Compiler.StackSpiller.RewriteMethodCallExpression(Expression expr, Stack stack) at System.Linq.Expressions.Compiler.StackSpiller.RewriteExpression(Expression node, Stack stack) at System.Linq.Expressions.Compiler.StackSpiller.ChildRewriter.Add(Expression expression) at System.Linq.Expressions.Compiler.StackSpiller.ChildRewriter.AddArguments(IArgumentProvider expressions) at System.Linq.Expressions.Compiler.StackSpiller.RewriteMethodCallExpression(Expression expr, Stack stack) at System.Linq.Expressions.Compiler.StackSpiller.RewriteExpression(Expression node, Stack stack) at System.Linq.Expressions.Compiler.StackSpiller.ChildRewriter.Add(Expression expression) at System.Linq.Expressions.Compiler.StackSpiller.ChildRewriter.AddArguments(IArgumentProvider expressions) at System.Linq.Expressions.Compiler.StackSpiller.RewriteMethodCallExpression(Expression expr, Stack stack) at System.Linq.Expressions.Compiler.StackSpiller.RewriteExpression(Expression node, Stack stack) at System.Linq.Expressions.Compiler.StackSpiller.RewriteExpressionFreeTemps(Expression expression, Stack stack) at System.Linq.Expressions.Compiler.StackSpiller.Rewrite[T](Expression1 lambda)
at System.Linq.Expressions.Expression1.Accept(StackSpiller spiller) at System.Linq.Expressions.Compiler.LambdaCompiler.Compile(LambdaExpression lambda) at System.Linq.Expressions.Expression1.Compile(Boolean preferInterpretation)
at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.CreateExecutorLambda[TResults]()
at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.CreateQueryExecutor[TResult](QueryModel queryModel)
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileQueryCore[TResult](Expression query, IQueryModelGenerator queryModelGenerator, IDatabase database, IDiagnosticsLogger1 logger, Type contextType) at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass13_01.<Execute>b__0()
at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQueryCore[TFunc](Object cacheKey, Func1 compiler) at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.Execute[TResult](Expression query) at Remotion.Linq.QueryableBase`1.System.Collections.IEnumerable.GetEnumerator() at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList(JsonWriter writer, IEnumerable values, JsonArrayContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty) at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.Serialize(JsonWriter jsonWriter, Object value, Type objectType) at Newtonsoft.Json.JsonSerializer.SerializeInternal(JsonWriter jsonWriter, Object value, Type objectType) at Newtonsoft.Json.JsonConvert.SerializeObjectInternal(Object value, Type type, JsonSerializer jsonSerializer) at My.api.MyController.MyServiceMethod(Guid projectId, DateTime fromDate, DateTime toDate) in C:\Sources[...]MyController.cs:line 347

---

因为这一切都在引擎盖下，我不知道如何处理这个问题。我也对解决方法或一些提示感到满意。

谁能帮帮我？

Answer 1

您的 projectEfforts 不是集合。这是一个查询，您正在运行它两次。第二次产生错误。

因此，您的解决方法是只运行一次查询，并从结果中得出 effHours，而不是将其作为第二个查询。

var projectEffortsQuery = from projectEntry in _context.ProjectTrackingEntry
                     where projectEntry.ProjectId == projectId
                     group projectEntry by new { projectEntry.Day.Year, projectEntry.Day.Month } into g
                     select new
                     {
                         PeDate = new DateTime(g.Key.Year, g.Key.Month, 1),
                         Month = g.Key,
                         MonthlyWorkingHours = g.Sum(x => x.WorkingHours)
                     };

var projectEfforts = projectEffortsQuery.ToList();