F＃中的实体框架和匿名类型 [英] Entity Framework and Anonymous Types in F#

问题描述

尝试使用匿名类型获取查询：

Trying to get a query with an anonymous type working:

let temporaryBookModel =
  query <@ context.Books 
    |> Seq.filter (fun book -> book.Id = bookId)
    |> Seq.map(fun item -> (item.Id, item.ParentUser.Id, item.ParentUser.Alias, item.Tagline, item.Title, item.Visible, item.CreatedDate))
    |> Seq.head @>

我不断得到：

{
LINQ to Entities中只支持无参数的构造函数和初始值设置。}

{"Only parameterless constructors and initializers are supported in LINQ to Entities."}

其中如果我将值直接映射到一个类型，那么匿名类型不应该抛出这个异常，因为它们是基于对象初始化器的功能的？不幸的是，我在匿名类型上找到的东西似乎说这是正确的语法。这样或这样的东西：

Which would make sense if I were mapping the values to a type directly, but anonymous types shouldn't throw this exception I would think since they are based on the object initializer functionality? Unfortunately anything I found on anonymous types seem to say this is the correct syntax. That or something like this:

let temporaryBookModel =
  query <@ context.Books 
    |> Seq.filter (fun book -> book.Id = bookId)
    |> Seq.map(fun item -> (("a", item.Id), ("b", item.ParentUser.Id), ("c", item.ParentUser.Alias), ("d", item.Tagline), ("e", item.Title, item.Visible), ("f", item.CreatedDate)))
    |> Seq.head @>

推荐答案

F＃是否支持匿名类型？ strong>

Does F# supports anonymous types?

我知道 - 它没有。但是有两种可能的解决办法：

As I know - it doesn't. But there are 2 possible ways for workarounds:

• 使用元组（正如你所使用的）


• 使用记录类型，但在这种情况下，您需要先定义记录。这样的事情：

type Book =
    { 
        Id: int;
        ParentId: int;
        ParentAlias: string;
        TagLine: string;
        Title: string;
        Visible: bool;
        CreatedDate: DateTime;
    }

使用代码行将如下所示：

And usage code line will looks like:

...
|> Seq.map 
    (fun item -> 
        {
            Id = item.Id; 
            ParentId = item.ParentUser.Id;
            ParentAlias = item.ParentUser.Alias;
            TagLine = item.Tagline;
            Title = item.Title;
            Visible = item.Visible;
            CreatedDate = item.CreatedDate
        }) 

您可以在类似的问题中找到更多的解释 here

More explanations you can find in similar question here

更新：

对于我来说，记录类型的使用是更优雅的解决方案，但是看起来它并不适用于Entity Framework - F＃ - >使用无参数构造函数进行记录？

Record types usage as for me is more elegant solution, BUT it looks like it doesn't not work with Entity Framework - F# -> Record with parameterless constructor?.

根据Tomas Petricek的回答，它必须被声明为具有较少构造函数和必要属性的参数的显式类型。

So according to Tomas Petricek answer - it has to be declared explicit type with parameters less constructor and necessary properties.

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