Is XQuery 3.1 designed for advanced JSON editing?

Question

XQuery 3.1 introduced several JSON functions. I was wondering if these functions were designed with advanced JSON editing in mind.

As far as I can tell, these functions only work for simple JSONs, like for instance...

let $json:={"a":1,"b":2} return map:put($json,"c",3)
{
  "a": 1,
  "b": 2,
  "c": 3
}

and

let $json:={"a":1,"b":2,"c":3} return map:remove($json,"c")
{
  "a": 1,
  "b": 2
}

The moment the JSON gets a bit more complex:

let $json:={"a":{"x":1,"y":2},"b":2} return map:put($json?a,"z",3)
{
  "x": 1,
  "y": 2,
  "z": 3
}

let $json:={"a":{"x":1,"y":2,"z":3},"b":2} return map:remove($json?a,"z")
{
  "x": 1,
  "y": 2
}

Obviously map:put() and map:remove() do exactly what you tell them to do; select the "a"-object and add or remove an attribute.
However, when I want to edit a JSON document, I'd like to edit the entire document. And as far as I know that's not possible with the current implementation. Or is it? At least something like map:put($json,$json?a?z,3) or map:remove($json,$json?a?z) doesn't work.

For the removal of the "z"-attribute I did come up with a custom recursive function (which only works in this particular use-case)...

declare function local:remove($map,$key){
  if ($map instance of object()) then
    map:merge(
      map:keys($map)[.!=$key] ! map:entry(.,local:remove($map(.),$key))
    )
  else
    $map
};
let $json:={"a":{"x":1,"y":2,"z":3},"b":2} return
local:remove($json,"z")

...with the expected output...

{
  "a": {
    "x": 1,
    "y": 2
  },
  "b": 2
}

...but I wasn't able to create a custom "add"-function.

I imagine advanced JSON editing can be done with some pretty advanced custom functions, but instead I would very much like to see that something like map:put($json,$json?a?z,3) would work, or otherwise an extra option which lets map:put() put out the entire JSON document, like map:put($json?a?z,3, ).

Or... I'd have to settle with the notion that XQuery isn't the right choice of course.

user_415448 · Answer

You're correct that doing what I call deep update of a map is quite difficult with XQuery 3.1 (and indeed XSLT 3.0) as currently defined. And it's not easy to define language constructs with clean semantics. I attempted to design a construct as an XSLT extension instruction - see https://saxonica.com/documentation10/index.html#!extensions/instructions/deep-update -- but I don't think its anywhere near a perfect solution.

user_4572117 · Answer

I wanted the same thing, so wrote my own surrogate XSLT functions, tan:map-put() and tan:map-remove(), which do deep map replacement and removal: https://github.com/Arithmeticus/XML-Pantry/tree/master/maps-and-arrays

These can be incorporated in an XSLT workflow via xsl:include or xsl:import, or in an XQuery one via fn:transform(). Some of the other functions may be useful, too. If these functions don't do exactly what you want, they might catalyze your own variation.

user_1501222 · Answer

In XQuery 3.1, you are supposed to write a recursive function for such things. You could put all you functions in a module file, and then load the module when you need them...

Besides that, Xidel has an object editing extension from before JSONiq and XPath 3.1. For a global mutable variable (without let), you can write:

$json:={"a":{"x":1,"y":2,"z":3},"b":2},
(($json).a).z := 4   


$json:={"a":{"x":1,"y":2,"z":3},"b":2},
$json("a")("z") := 4

user_2703456 · Answer

#comment119588453_67652693 by @ChristianGrün:

If updates are required, we tend to convert JSON data to XML.

I'm a xidel user and last week (with a little help from @BeniBela) I've had a look at whether this could be done with json-to-xml(), Xidel's own x-replace-nodes() and xml-to-json(). The answer is yes. Thanks for the hint.

For reference and for anyone interested, here's 1 example.

To change key "c" in {"x":{"a":1,"b":2,"c":3},"y":2} to "d":

$ xidel -s '{"x":{"a":1,"b":2,"c":3},"y":2}' -e '
  xml-to-json(
    x:replace-nodes(
      json-to-xml(
        serialize($json,{"method":"json"})
      )//fn:map[@key="x"]/fn:number[@key="c"]/@key,
      attribute key {"d"}
    )
  )
'
{"x":{"a":1,"b":2,"d":3},"y":2}

Xidel online tester.

Is XQuery 3.1 designed for advanced JSON editing?

4 Answers