[TypedMongo] Allowing the discriminator field at any position

[pulewicz]

What is the easiest way of reading class hierarchies with discriminator fields at the end of the object? Unfortunately, we need to adapt to an existing schema.

As explained in https://github.com/AVSystem/scala-commons/blob/master/docs/GenCodec.md#flat-sealed-hierarchy-format:

It may also be sensitive to object field order because the discriminator field must be read before any other fields (this is only a problem with Input implementations that do not support random field access by name - JsonStringInput does not have this problem).

I did some checks, and it seems like BsonReaderInput has this problem, but BsonValueInput hasn't: https://github.com/pulewicz/mongo-commons-playground/blob/master/src/main/scala/PlaygroundApp.scala#L47

Questions:

1. Is there any easy way to use TypedMongoCollection with value input and output? From what I see currently it's hardcoded and it will need creating your own versions of the following classes, is it correct?

   • https://github.com/AVSystem/scala-commons/blob/master/commons-mongo/jvm/src/main/scala/com/avsystem/commons/mongo/GenCodecBasedBsonCodec.scala#L14
   • https://github.com/AVSystem/scala-commons/blob/master/commons-mongo/jvm/src/main/scala/com/avsystem/commons/mongo/core/GenCodecProvider.scala#L10
   • https://github.com/AVSystem/scala-commons/blob/master/commons-mongo/jvm/src/main/scala/com/avsystem/commons/mongo/core/GenCodecRegistry.scala#L14
   • https://github.com/AVSystem/scala-commons/blob/master/commons-mongo/jvm/src/main/scala/com/avsystem/commons/mongo/typed/TypedMongoCollection.scala#L35

2. Is using the value input and output a good solution, or are there any disadvantages?

Regards,
Paweł Ulewicz.

[ghik]

Hi,

Thanks for the report. I'll see if I can make BsonReaderInput independent of field order similarly to how it's done in JsonStringInput. Otherwise I will look for ways make it possible to use BsonValueInput in TypedMongoCollection. The primary advantage of BsonReaderInput over BsonValueInput is performance - it avoids the intermediate BsonValue representation.

[pulewicz]

Hi,

Great to hear that and thanks for help. Please contact me if you need more information.

Regards,
Paweł Ulewicz.

[ghik]

Fixed in 2.6.0

[pulewicz]

Hi,

Thanks for the info, but how to use the fix? I updated the version to 2.6.0 in my playground app and the BsonReaderInput example still doesn't work: https://github.com/pulewicz/mongo-commons-playground/blob/master/src/main/scala/PlaygroundApp.scala#L62

com.avsystem.commons.serialization.GenCodec$MissingCase: Cannot read field x of HierarchyTypes.TestType before type field is read

Should the reader input work now or should I use the value reader instead? I checked the code in the PR, but there doesn't seem to be anything related to the issue.

Regards,
Paweł Ulewicz.

[ghik]

An ObjectInput implementation is made immune to field order changes by implementing peekField. You can see that done here. Unfortunately, it doesn't work for BsonDocumentReader because of an upstream bug (which I did not manage to properly minimize and report yet).

I assumed that this shouldn't be a problem for users of TypedMongoCollection because it uses BsonBinaryReader internally (via GenCodecRegistry). Do you actually need to use BsonDocumentReader besides this test? Note that if you want to read BsonDocuments then it should be more straightforward to just use BsonValueInput.

[pulewicz]

Hi,

I used the BsonDocumentReader just for this example for simplicity. I'll try the real code then. Thanks for the info.

Regards,
Paweł Ulewicz.

[pulewicz]

It works, thanks!

Regards,
Paweł Ulewicz.