F #: In LINQ, only constructors and initializers without parameters are supported for objects

Question

I am trying to return Envelope from a request. The envelope is defined as follows.

[<CLIMutable>]
type Envelope<'T> = {
    Id : Guid
    StreamId: Guid
    Created : DateTimeOffset
    Item : 'T }

MyLibAAS.DataStore.MyLibAASDbContext is an EF DbContext written in C #. When I extend it to f # as follows, I get an error:Only parameterless constructors and initializers are supported in LINQ to Entities.

type MyLibAAS.DataStore.MyLibAASDbContext with 
    member this.GetEvents streamId = 
        query {
            for event in this.Events do
            where (event.StreamId = streamId)
            select {
                Id = event.Id;
                StreamId = streamId;
                Created = event.Timestamp;
                Item = (JsonConvert.DeserializeObject<QuestionnaireEvent> event.Payload)
            }
        } 

If I get my event back and list it in Envelope after the fact, it works fine.

type MyLibAAS.DataStore.MyLibAASDbContext with 
    member this.GetEvents streamId = 
        query {
            for event in this.Events do
            where (event.StreamId = streamId)
            select event
        } |> Seq.map (fun event ->
                {
                    Id = event.Id
                    StreamId = streamId
                    Created = event.Timestamp
                    Item = (JsonConvert.DeserializeObject<QuestionnaireEvent> event.Payload)
                }
            )

Why does it matter? The type of envelope is not even type EF.

Answer 1

F #
F # .NET- , ( ).
, # :

public class Envelope<T> : IComparable<Envelope<T>>, IEquatable<Envelope<T>>, ...
{
   public Guid Id { get; private set; }
   public Guid StreamId { get; private set; }
   public DateTimeOffset Created { get; private set; }
   public T Item { get; private set; }

   public Envelope( Guid id, Guid streamId, DateTimeOffset created, T item ) {
      this.Id = id;
      this.StreamId = streamId;
      this.Created = created;
      this.Item = item;
   }

   // Plus implementations of IComparable, IEquatable, etc.
}

F #, F # , .
, select # :

select new Envelope<QuestionnaireEvent>( 
   event.Id, streamId, event.Timestamp, 
   JsonConvert.DeserializeObject<QuestionnaireEvent>(event.Payload) )

Entity Framework
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from e in ...
let env = new Envelope<E>( e.Id, ... )
where env.Id > 0
select env

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( : # , EF )

, Envelope . - , :

type Envelope<'T>() =
   member val Id : Guid = Guid.Empty with get, set
   member val StreamId : Guid = Guid.Empty with get, set
   member val Created : DateTimeOffset = DateTimeOffset.MinValue with get, set
   member val Item : 'T = Unchecked.defaultof<'T> with get, set

:

select Envelope<_>( Id = event.Id, StreamId = streamId, ... )

select Seq.map
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,
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Answer 2

LINQ to Entities, , query, , . .

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