Deep JSON merge - json

Question

I have multiple JSON files that I'd like to merge into one.
Some have the same root element but different children. I don't want to overwrite the children but too extend them if they have the same parent element.
I've tried this answer, but it doesn't work:
jq: error (at file2.json:0): array ([{"title":"...) and array ([{"title":"...) cannot be multiplied
Sample files and wanted result (Gist)
Thank you in advance.

Answer 1

Here is a recursive solution which uses group_by(.key) to decide
which objects to combine. This could be a little simpler if .children
were more uniform. Sometimes it's absent in the sample data and sometimes it's the unusual value [{}].
def merge:
def kids:
map(
.children
| if length<1 then empty else .[] end
)
| if length<1 then {} else {children:merge} end
;
def mergegroup:
{
title: .[0].title
, key: .[0].key
} + kids
;
if .==[{}] then .
else group_by(.key) | map(mergegroup)
end
;
[ .[] | .[] ] | merge
When run with the -s option as follows
jq -M -s -f filter.jq file1.json file2.json
It produces the following output.
[
{
"title": "Title1",
"key": "12345678",
"children": [
{
"title": "SubTitle2",
"key": "123456713",
"children": [
{}
]
},
{
"title": "SubTitle1",
"key": "12345679",
"children": [
{
"title": "SubSubTitle1",
"key": "12345610"
},
{
"title": "SubSubTitle2",
"key": "12345611"
},
{
"title": "DifferentSubSubTitle1",
"key": "12345612"
}
]
}
]
}
]
If the ordering of the objects within the .children matters
then an a sort_by can be added to the {children:merge} expression,
e.g. {children:merge|sort_by(.key)}

Answer 2

Here is something that will reproduce your desired result. It's by no means automatic, It's really a proof of concept at this stage.
One liner:
jq -s '. as $in | ($in[0][].children[].children + $in[1][].children[0].children | unique) as $a1 | $in[1][].children[1] as $s1 | $in[0] | .[0].children[0].children = ($a1) | .[0].children += [$s1]' file1.json file2.json
Multi line breakdown (Copy/Paste):
jq -s '. as $in
| ($in[0][].children[].children + $in[1][].children[0].children
| unique) as $a1
| $in[1][].children[1] as $s1
| $in[0]
| .[0].children[0].children = ($a1)
| .[0].children += [$s1]' file1.json file2.json
Where:
$in : file1.json and file2.json combined input
$a1: merged "SubSubTitle" array
$s1: second subtitle object
I suspect the reason this didn't work was because your schema is different and has nested arrays.
I find it quite hypnotic looking at this, it would be good if you could elaborate a bit on how fixed the structure is and what the requirements are.