Question

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 @>

And I keep getting:

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

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 @>

Answer 1

Does F# supports anonymous types?

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

• use tuples (as you’re using)
• use record types, but in this case you’ll need to define record before. Something like this:

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

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
        }) 

More explanations you can find in similar question here

Update:

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?.

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