f# Powerpack和实体框架:如何重构两个查询的公共部分

Question

我有两个查询:

 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 @>

我想重构它，使这两个

 fun currentUser -> currentUser.Id = userId

可以是一个方法，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)) @>

但是我一直得到一个错误:

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

这让我觉得我的方法签名构造得很差。作为f#的新手，这让我有点困惑，因为我确信这可以在c#中使用返回Func的方法来完成。然而，我理解这是有区别的，因为f#使用不同的库来构造linq查询。

Answer 1

如果您将方法更改为返回Expr，它是否工作

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

和使用拼接?

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