linq F#带有多个条件的左联接，作为查询表达式

vwkv1x7d  于 2023-02-01  发布在  其他
关注(0)|答案(1)|浏览(138)

在C#中，我可以使用多个条件进行左连接查询，就像下面这个C#示例运行良好：

using (var db = new CompanyContext())
{
    var q =
        from d in db.deps
        from e in db.emps.Where(e => d.id==e.dep_id && d.start_time<e.modified).DefaultIfEmpty() // left join
        select new { d, e };
    var result = q.ToList();
    // ...
}

我试着在F#中把它翻译成下面的形式：

use db = new CompanyContext()
let q = query {
    for d in db.deps do
    for e in db.emps.Where(fun e -> d.id=e.dep_id && d.start_time<e.modified).DefaultIfEmpty() do // left join
    select ( d, e )}
let result = q.ToList()
// ...

但F#版本会导致一个异常：
系统操作无效异常：从范围""引用了类型为"MyLib. dep"的变量"_arg1"，但未定义该变量
我的F#查询有什么问题？

    • EDIT**：使用匿名记录select {| d = d; e = e |}而不是建议的select ( d, e )，会导致相同的异常
    • EDIT**：完整异常（没有内部异常。它为null）：
System.InvalidOperationException: variable '_arg1' of type 'MyLib.dep' referenced from scope '', but it is not defined
   at System.Linq.Expressions.Compiler.VariableBinder.Reference(ParameterExpression node, VariableStorageKind storage)
   at System.Linq.Expressions.Compiler.VariableBinder.VisitParameter(ParameterExpression node)
   at System.Linq.Expressions.Compiler.VariableBinder.Visit(Expression node)
   at System.Linq.Expressions.Compiler.VariableBinder.VisitUnary(UnaryExpression node)
   at System.Linq.Expressions.Compiler.VariableBinder.Visit(Expression node)
   at System.Linq.Expressions.ExpressionVisitor.Visit(ReadOnlyCollection`1 nodes)
   at System.Linq.Expressions.ExpressionVisitor.VisitNewArray(NewArrayExpression node)
   at System.Linq.Expressions.Compiler.VariableBinder.Visit(Expression node)
   at System.Linq.Expressions.ExpressionVisitor.VisitArguments(IArgumentProvider nodes)
   at System.Linq.Expressions.ExpressionVisitor.VisitMethodCall(MethodCallExpression node)
   at System.Linq.Expressions.Compiler.VariableBinder.Visit(Expression node)
   at System.Linq.Expressions.ExpressionVisitor.VisitArguments(IArgumentProvider nodes)
   at System.Linq.Expressions.ExpressionVisitor.VisitMethodCall(MethodCallExpression node)
   at System.Linq.Expressions.Compiler.VariableBinder.Visit(Expression node)
   at System.Linq.Expressions.ExpressionVisitor.Visit(ReadOnlyCollection`1 nodes)
   at System.Linq.Expressions.Compiler.VariableBinder.VisitLambda[T](Expression`1 node)
   at System.Linq.Expressions.Compiler.VariableBinder.Visit(Expression node)
   at System.Linq.Expressions.Compiler.LambdaCompiler.Compile(LambdaExpression lambda, DebugInfoGenerator debugInfoGenerator)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.GetLambdaExpression(Expression argument)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.GetLambdaExpression(MethodCallExpression callExpression, Int32 argumentOrdinal)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.OneLambdaTranslator.Translate(ExpressionConverter parent, MethodCallExpression call, DbExpression& source, DbExpressionBinding& sourceBinding, DbExpression& lambda)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.OneLambdaTranslator.Translate(ExpressionConverter parent, MethodCallExpression call)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.TypedTranslate(ExpressionConverter parent, MethodCallExpression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.DefaultIfEmptyTranslator.Translate(ExpressionConverter parent, MethodCallExpression call)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.TypedTranslate(ExpressionConverter parent, MethodCallExpression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateLambda(LambdaExpression lambda, DbExpression input)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.OneLambdaTranslator.Translate(ExpressionConverter parent, MethodCallExpression call, DbExpression& source, DbExpressionBinding& sourceBinding, DbExpression& lambda)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.SelectManyTranslator.Translate(ExpressionConverter parent, MethodCallExpression call)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.TypedTranslate(ExpressionConverter parent, MethodCallExpression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.OneLambdaTranslator.Translate(ExpressionConverter parent, MethodCallExpression call, DbExpression& source, DbExpressionBinding& sourceBinding, DbExpression& lambda)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.SelectTranslator.Translate(ExpressionConverter parent, MethodCallExpression call)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.TypedTranslate(ExpressionConverter parent, MethodCallExpression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.Convert()
   at System.Data.Entity.Core.Objects.ELinq.ELinqQueryState.GetExecutionPlan(Nullable`1 forMergeOption)
   at System.Data.Entity.Core.Objects.ObjectQuery`1.<>c__DisplayClass41_0.<GetResults>b__1()
   at System.Data.Entity.Core.Objects.ObjectContext.ExecuteInTransaction[T](Func`1 func, IDbExecutionStrategy executionStrategy, Boolean startLocalTransaction, Boolean releaseConnectionOnSuccess)
   at System.Data.Entity.Core.Objects.ObjectQuery`1.<>c__DisplayClass41_0.<GetResults>b__0()
   at System.Data.Entity.Core.Objects.ObjectQuery`1.GetResults(Nullable`1 forMergeOption)
   at System.Data.Entity.Core.Objects.ObjectQuery`1.<System.Collections.Generic.IEnumerable<T>.GetEnumerator>b__31_0()
   at System.Data.Entity.Internal.LazyEnumerator`1.MoveNext()
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at ConsoleApplication1.main(String[] argv) in C:\Temp\ConsoleApplication1\ConsoleApplication1\Program.fs:line 780

groupJoin似乎不能用我的额外条件编译。Visual Studio抱怨它不知道d，并在d.start_time<e.modified中给它加下划线：

use db = new CompanyContext()
let q = query {
    for d in db.deps do
    groupJoin e in db.emps.Where(fun e -> d.start_time<e.modified) on (d.id=e.dep_id) into es // left join
    for e in es.DefaultIfEmpty() do
    select ( d, e )}
let result = q.ToList()

FS0039未定义值、命名空间、类型或模块"d"。

linq

1条答案

按热度按时间
gijlo24d

gijlo24d1#

我想我已经找到了一个解决方案。这似乎产生了预期的结果，groupJoin：

use db = new CompanyContext()
let q = query {
    for d in db.deps do
    groupJoin e in db.emps on (d.id=e.dep_id) into es
    for e in es.Where(fun e -> d.start_time<e.modified).DefaultIfEmpty() do
    select ( d , e )}
let result = q.ToList()

您不必使用on子句，而是使用一个始终为真的伪条件，如0=0：

use db = new CompanyContext()
let q = query {
    for d in db.deps do
    groupJoin e in db.emps on (0=0) into es
    for e in es.Where(fun e -> d.id=e.dep_id && d.start_time<e.modified).DefaultIfEmpty() do
    select ( d , e )}
let result = q.ToList()

这些示例也适用于leftOuterJoin而不是groupJoin。
意见：
我不喜欢leftOuterJoin隐式地将.DefaultIfEmpty()应用于into部分声明的内容因为如果你想在后面的for子句中添加更多的条件，就像我用.Where一样，那么你仍然需要应用.DefaultIfEmpty()，这样无论如何至少又有一个结果，因为for是下一个单独的操作，所以最好使用始终需要.DefaultIfEmpty()的groupJoin来执行左连接

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