entity-framework - F# Powerpack 和实体框架：如何重构两个查询的公共部分

Question

我有两个疑问：

 member private x.CheckIfUserExistsUsingId(userId:int, context:StoryBoardContext) =
  <@ context.Users 
     |> Seq.exists (fun currentUser -> currentUser.Id = userId) @>

和

member private x.FindUserById(userId:int, context:StoryBoardContext) =
 <@ context.Users 
    |> Seq.filter(fun currentUser -> currentUser.Id = userId) 
    |> Seq.head @>

我想重构这个，这样两个

 fun currentUser -> currentUser.Id = userId

可以是一种方法，例如：

member private x.IfUserIdMatches (userId:int) = 
  fun (currentUser:User) -> currentUser.Id = userId

然后使用它：

member private x.CheckIfUserExistsUsingId(userId:int, context:StoryBoardContext) =
  <@ context.Users 
     |> Seq.exists (x.IfUserIdMatches (userId)) @>

但我不断收到错误消息：

The following construct was used in query but is not recognised by the F#-to-LINQ query translator...

这让我觉得我对该方法的签名结构很差。作为 F# 的新手，我有点困惑，因为我很肯定这可以在 C# 中使用返回 Func 的方法来完成。但是，我知道由于 F# 使用不同的库来构建 linq 查询，因此存在差异。

Answer 1

如果您更改返回方法是否有效Expr

member private x.IfUserIdMatches (userId:int) = 
  <@ fun (currentUser:User) -> currentUser.Id = userId @>

并使用拼接？

member private x.CheckIfUserExistsUsingId(userId:int, context:StoryBoardContext) =
  <@ context.Users 
      |> Seq.exists %(x.IfUserIdMatches (userId)) @>