How to modify a BSON document

[Vin]

Hi,

I'm using bsoncxx and I'm having a hard time to understand how to modify a BSON document.

let's say I have this input :

 1) a JSON document  like :

{

 "name" : "john",

 "address" : 

  {

    "number" : "235",

    "Town" : " New York city"

  }

}

2) a rule set of modification directives to the BSON document :

a) convert value of address.number from string to int32

b) add the field address.Country with value "USA" to the document.

c) remove the field address.Town from the document.

from https://mongodb.github.io/mongo-cxx-driver/api/current/namespacebsoncxx.html I understand I can convert my JSON std::string to a BSON using

the instruction 

  document::value myBsonDoc = bsoncxx::from_json(myJsonString);

but then, what to do next with myBsonDoc for each of the actions needed in a) b) and c) ?

Thank you.

[Vin]

What's the solution the Mongo team would choose for this ?

Should I parse my json string with a third party JSON library (like Poco::Json), do the modification job pointed in 2a), 2b), 2c) then convert back my Json object to string then use 

document::value myBsonDoc = bsoncxx::from_json(myModifiedJsonString);

?

Thank you for your help.

[Sam Rossi]

Hi Vin! There currently isn't any way to modify BSON documents in bsoncxx; internally, the documents are only represented as an array of bytes, and we use libbson to read from it. Without using any third party libraries, the most idiomatic way to do what you want would be to parse the string with bsoncxx::from_json and then to recurse over it and build up a new document, making changes wherever necessary.

Let me know if you have any other questions or I didn't explain this clearly, and thanks for using the C++ driver!

[Vincent Dautremont]

Thank you Sam,

That's what I thought, and I've used bsoncxx::builder::stream::document to build a new document.

But I must say it's a bit sad to miss such a feature where you would just express the diff/changes you want from a base bsoncxx::document::value, and later ask the driver to generate a new document with these changes included.

I hope future updates of the driver will go in this direction.

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[Andrew Morrow]

Adding support for mutation to/above a BSON library is tricky to get right.

If you are interested in efficiency, then you most likely don't want to first de-serialize the whole document into a complete tree of objects scattered all over the heap, then apply mutations, and then re-serialize. Instead, you want something more like copy-on-write, where the underlying BSON buffer remains contiguous, and mutations are stored on the side. The server codebase does have such a facility (in particular, see document.cpp in that directory), built above its internal BSON library, but I'm not aware of any drivers that currently offer similar functionality.

It may be the case that you could build an analogous abstraction above the bsoncxx library, though I haven't thought too hard about how to do it or whether it is in fact possible given the current API to bsoncxx.

Thanks,

Andrew

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[Sam Rossi]

Interestingly, the functionality you describe (expressing a diff and generating a new document) is something we recently found necessary to implement in our testing code, although it's currently only on master (i.e. not released) and isn't part of the public API. If you're interested in seeing how we solved the problem, you can see it here (and in the corresponding .cpp file). We're not sure yet if there are use cases that it isn't flexible enough to handle (hence not releasing it as part of the API), but feel free to draw inspiration from it if you'd like to implement something like that for your own use.

[Vincent Dautremont]

Thank you, that's really interesting,

I'll reuse this code for sure if I have new transformation needs.

From what I've read, I'll just need to add code to handle transformation that adds K+V to a Document (and V to an Array).

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[Sam Rossi]

Yep! Each key/value pair will be passed into the function you provide, and whatever you return will be appended in its place (so you just return what was passed in if you don't want to change it). stdx::optional::nullopt will be passed in as the key for array values, and if the value you're returning is non-owning, append it to the array pointer passed in so it lives long enough.

You can see some examples of how we use it here, although they're somewhat complex, so it might not be super clear what the transformations are intended to do.