[Question] Hook/intercep before deserializing json document

[sszakal]

Hi all,

Is there a way in MartenDB to intercept a JSON document before it gets converted to CLR object? (I'm currently on Marten V3).

The reason I'm interested in this type of hook/callback is to apply on-the-fly migrations/transformations to the JSON. I know marten supports JSON transformations using the patch API or using javascript transformations, but these are executed on the database side and it doesn't scale very well and, somehow, limited. Running transformations on the .NET side would allow much more powerful and complex transformations to be performed.

Please let me know what options do I have?

Thanks,
Stef

[mysticmind]

https://martendb.io/diagnostics.html#listening-for-document-store-events - Check DocumentLoaded and see if it works for your use case.

https://martendb.io/v3/documentation/documents/diagnostics - v3 docs pertaining to IDocumentSessionListener

Edit: I don't think you can modify the loaded docs. This will need to be an enhancement to deal with.

[sszakal]

Thanks @mysticmind .
Yes, IDocumentSessionListener allows to intercept the document before load, but not before it gets deserialized.

Are there any plans for adding such hook in IDocumentSessionListener ?
If I would add such feature myself and open a pull request, where would be the best place to intercept the json stream ?
I just had a quick look at the source code and I can see few possible places:IdentityMap or , and probably better, in DocumentStorage (Resolve, Fetch methods)

Thanks,
Stef

[mysticmind]

@sszakal we are happy to take a PR. Note that 3.x is in maintenance mode so we are not adding any new features/enhancements. This feature will need to be in the latest 5.x.

@jeremydmiller Could you please provide some pointers for Stefan to take look at this?

[jeremydmiller]

@oskardudycz is working on event upcasters now that will be in 5.9 pretty soon.

[oskardudycz]

The PR #2314 is merged already, @sszakal, have a look at the upcoming feature docs: https://630b6e791b8fa54aba654d0a--lucid-northcutt-013a56.netlify.app/events/versioning.html. It'll allow performing raw JSON transformations both for Newtonsoft Json.NET and System.Text.Json.

[sszakal]

Thanks @oskardudycz I’m looking forward for this feature !

[oskardudycz]

@sszakal, I think that 5.9 should be released pretty soon, we'll ping you when it's out :)

[oskardudycz]

@sszakal 5.9.0 is out, see the docs https://martendb.io/events/versioning.html :)

[oskardudycz]

In version 5.9.0, we added Upcaster functionality that allows transforming events' JSON payloads on the fly. Read more in https://martendb.io/events/versioning.html#upcasting-advanced-payload-transformations 🙂