Question

使用SqlProvider类型提供程序，我正在尝试做一些事情，我递归地折叠一个'查询标准'列表，

type Criterion = {
    Column : string
    Operator : string
    Value : string
}

这样表达式树只被编译成SQL一次，而且我没有多次访问数据库。我尝试了一些方法，其中最成功的方法是这样的

let rec eval (acc : IQueryable<SourceEntity> option) (qrys : Criterion list) =
match qrys with
|[] -> acc
|x :: xs -> let acc' = let op,valu = translateOpnValu x
                       match acc with
                       |Some acc' -> query {
                                            for elem in acc' do
                                            where (elem.GetColumn x.Column op valu)
                                            select elem 
                                     } |> Some
                       |None     -> query {
                                            for elem in ctx.Dbo.Source do
                                            where (elem.GetColumn x.Column op valu)
                                            select elem 
                                     } |> Some
            eval acc' xs

函数translateOpnValu是

let translateOpnValu (c:Criterion) =
     match c.Operator with
     |"%=%" -> (=%), sprintf "%%%s%%" c.Value
     |_     -> (=), c.Value

我得到了这个赘述

System.Exception: Unsupported expression. Ensure all server-side objects appear on the left hand side of predicates.  The In and Not In operators only support the inline array syntax. InvokeFast(elem.GetColumn("Source Code"), value(FSI_0006+acc'@38-2), "%BEN%")
at Microsoft.FSharp.Linq.RuntimeHelpers.LeafExpressionConverter.EvaluateQuotation(FSharpExpr e)
at Microsoft.FSharp.Linq.QueryModule.EvalNonNestedInner(CanEliminate canElim, FSharpExpr queryProducingSequence)
at Microsoft.FSharp.Linq.QueryModule.EvalNonNestedOuter(CanEliminate canElim, FSharpExpr tm)
at Microsoft.FSharp.Linq.QueryModule.clo@1735-1.Microsoft-FSharp-Linq-ForwardDeclarations-IQueryMethods-Execute[a,b](FSharpExpr`1 )
at FSI_0006.evaluate(FSharpOption`1 acc, FSharpList`1 qrys) in F:\code_root\vs2015\F\CAMS\CAMS\scratch.fsx:line 47
at <StartupCode$FSI_0007>.$FSI_0007.main@() in F:\code_root\vs2015\F\CAMS\CAMS\scratch.fsx:line 60

如果我用translate隐式运算符（= / =％）替换translateOpnValu返回的'op'，它可以正常工作。

我感觉这与被返回的运算符的类型被约束为（string - ＆gt; string - ＆gt; bool）的事实有关，而隐式运算符更通用。我怎样才能获得translateOpnValu函数来返回更多通用运算符？或许这根本不是问题......

Answer 1

@Fyodor是正确的 - 为了让SQL提供程序正确地获取你的函数，你需要将它包装在一个引号中并将其拼接到查询表达式中。这样的事情应该有效：

let translateOpnValu (c:Criterion) =
     match c.Operator with
     |"%=%" -> <@ (=%) @>, sprintf "%%%s%%" c.Value
     |_     -> <@ (=) @>, c.Value

// ...

query {
    for elem in acc' do
    where ((%op) (elem.GetColumn x.Column) valu)
    select elem 
}

SqlProvider以递归方式编写查询

1 个答案: