MongoListField saves list of case class instances as strings

[Erik Allik]

When saving a list of case class instances into a MongoListField, they're saved to the DB as strings, and also loaded back as such.

This seems to be a bug. Furthermore, I'm also of the opinion that a framework written in a statically typed language should be able to produce a compilation error in cases like these, unless there's some extraneous dynamism going on in its core, but I also acknowledge that I'm in no position to criticize how the Record framework's been designed.

The code to reproduce the issue is attached, or can be viewed at http://pastebin.com/haahzKYf

Regards,

Erik Allik

[Antonio Salazar Cardozo]

Wat. So what we're saying here is when you load it back you actually get a list of String instead

of a list of Bar? But at compile time it essentially appears to be a List of Bar?

For what it's worth, if that's what we're talking about, (a) there is extraneous dynamism (populating

a list from an external entity where the objects have been serialized at runtime is not type-checked

at compile time, because it pretty much has to use reflection unless it uses Java serialization, which

is super-brittle and nasty) and (b) we don't get a runtime error at deserialization time because the

JVM erases types at runtime, so there's no check on the types of objects that go into a collection,

even if their types were specified at compile-time.

Thanks,

Antonio

[Tim Nelson]

MongoListField only supports basic types. If you want to use case classes you need to use the MongoCaseClassListField, or as I prefer, BosnRecords with BsonRecordListField.

Tim

[Antonio Salazar Cardozo]

So the followup question is, is there a way that we can encode that in the type system so you don't find

yourself accidentally using MogoListField when you want MongoCaseClassListField? What does “basic

types” mean? Can we require a T <: AnyVal or something like that?

Thanks,

Antonio

[Tim Nelson]

I've been wondering if there was a way to basically require a type to be one from a list of types.

The types that are supported are those checked in the asDBObject function [1]. The functions are defined in Meta.scala [2].

There are a bunch of Java types that are supported, so I don't think AnyVal would work. 

Tim

1 - https://github.com/lift/framework/blob/master/persistence/mongodb-record/src/main/scala/net/liftweb/mongodb/record/field/MongoListField.scala#L115

2 - https://github.com/lift/framework/blob/master/persistence/mongodb/src/main/scala/net/liftweb/mongodb/Meta.scala

[Antonio Salazar Cardozo]

There are some small tricks to create type unions, as per http://www.chuusai.com/2011/06/09/scala-union-types-curry-howard/:

  type ¬[T] = T => Nothing
  type ¬¬[T] = ¬[¬[T]]
  type ∧[T, U] = T with U
  type ∨[T, U] = ¬[¬[T] ∧ ¬[U]]

Towards the end of the post, you see how we would implement a signature with implicit parameter that

would require the type to be one of a list of types. Not 100% sure this is a good idea (it certainly makes

the code a bit harder to follow), and I'd be curious to hear others' thoughts on this.

In this case it definitely looks like it could be handy though.

Thanks,

Antonio