Using JQ to inject element at specific position based on conditions - json

Question

Pulling my hair here, trying to use jq to parse and extend a JSON file and adding an element at a specific position based on a certain condition.
Here is a sample file (also at https://jqplay.org/s/6cjmbnvrqu)
{
"TopA": { "stuff": "here"},
"TopB": {
"C591AB7E": {
"Type": "this",
"Properties": {
"lots": 1,
"of": 2,
"values" : 3
}
},
"7E16765A": {
"Type": "this",
"Properties": {
"lots": 4,
"of": 5,
"values" : 6
}
},
"AAD76465": {
"Type": "that",
"Properties": {
"lots": 7,
"of": 8,
"values" : 9
}
}
}
}
The goal is to add an element to the Properties node of any TopB child where .Type == "that". And the kicker is that I need to put the child node's key into the new element value with an added prefix.
So essentially I need the last element to look like this:
"AAD76465": {
"Type": "that",
"Properties": {
"lots": 7,
"of": 8,
"values": 9,
"newElement": "Prefix-AAD76465"
}
}
I also need to retain the whole rest of the file (or a new file for that matter). So I don't need a query but really a jq call to manipulate the existing file. Parallel to TopB there could be other elements that I'd still need in the file. And no, I don't know, neither do I have control over the naming of the children of TopB. All I have is that my targets are nested children of TopB with .Type == "that". There can be multiple of those.
Thanks for looking.

Answer 1

If you get the path to where you will be adding the new field first, you can simply extract the parent key from that.
reduce path(.TopB[] | select(.Type == "that") .Properties.newElement) as $p (.; setpath($p; "Prefix-\($p[1])"))
Online demo

Answer 2

Here'a solution that uses walk/1 and is therefore perhaps quite intuitive:
walk(if (type == "object") and .TopB
then .TopB |= with_entries(
.key as $key
| if .value.Type == "that"
then .value.Properties += { newElement: ("Prefix-" + $key) }
else . end)
else . end)
One advantage of this solution is that it meets the stated requirement:
The goal is to add an element to the Properties node of any TopB child ...
in the sense that no assumptions are made about the locations of objects with the "topB" key.