How to get a subobject out of JSON using jq, keeping final key in the result without Bash processing? - json

Question

I'm writing a Bash function to get a portion of a JSON object. The API for the function is:
GetSubobject()
{
local Filter="$1" # Filter is of the form .<key>.<key> ... .<key>
local File="$2" # File is the JSON to get the subobject
# Code to get subobject using jq
# ...
}
To illustrate what I mean by a subobject, consider the Bash function call:
GetSubobject .b.x.y example.json
where the file example.json contains:
{
"a": { "p": 1, "q": 2 },
"b":
{
"x":
{
"y": { "j": true, "k": [1,2,3] },
"z": [4,5,6]
}
}
}
The result from the function call would be emitted to stdout:
{
"y": {
"j": true,
"k": [
1,
2,
3
]
}
}
Note that the code jq -r "$Filter" "$File" would not give the desired answer. It would give:
{ "j": true, "k": [1,2,3] }
Please note that the answer I'm looking for needs to be something I can use in the Bash function API above. So, the answer should use the Filter and File variables as show above and not be specific to the example above.
I have come up with a solution; however, it relies on Bash to do part of the job. I am hoping that the solution can be pure jq without reliance on Bash processing.
#!/bin/bash
GetSubobject()
{
local Filter="$1"
local File="$2"
# General case: separate:
# .<key1>.<key2> ... .<keyN-1>.<keyN>
# into:
# Prefix=.<key1>.<key2> ... .<keyN-1>
# Suffix=<keyN>
local Suffix="${Filter##*.}"
local Prefix="${Filter%.$Suffix}"
# Edge case: where Filter = .<key>
# Set:
# Prefix=.
# Suffix=<key>
if [[ -z $Prefix ]]; then
Prefix='.'
Suffix="${Filter#.}"
fi
jq -r "$Prefix|to_entries|map(select(.key==\"$Suffix\"))|from_entries" "$File"
}
GetSubobject "$#"
How would I complete the above Bash function using jq to obtain the desired result, hopefully in a less brute-force way that takes advantage of jq's capabilities without having to do pre-processing in Bash?

Answer 1

Somewhat further simplifying the jq part but with the same general constraints as JawguyChooser's answer, how about the much simpler Bash function
GetSubject () {
local newroot=${1##*.}
jq -r "{$newroot: $1}" "$2"
}
I may be overlooking some nuances of your more-complex Bash processing, but this seems to work for the example you provided.

Answer 2

If I understand what you're trying to do correctly, it doesn't seem possible to me to do it "pure jq" having read the docs (and being a regular jq user myself). The closest I could come to helping here was to simplify the jq part itself:
jq -r "$Prefix| { $Suffix }" "$File"
This has the same behavior as your example (on this limited set of cases):
GetSubobject '.b.x.y' example.json
{
"y": {
"j": true,
"k": [
1,
2,
3
]
}
}
This is really a case of metaprogramming, you want to programmatically operate on a jq program. Well, it makes sense (to me) that jq takes its program as input but doesn't allow you to alter the program itself. bash seems like an appropriate choice for doing the metaprogramming here: to convert a jq program into another one and then run jq using that.

Answer 3

If the goal is to do as little as possible in bash, then maybe the following bash function will fill the bill:
function GetSubobject {
local Filter="$1" # Filter is of the form .<key>.<key> ... .<key>
local File="$2" # File is the JSON to get the subobject
jq '(null|path('"$Filter"')) as $path
| {($path[-1]): '"$Filter"'}' "$File"
}
An alternative would be to pass $Filter in as a string (e.g. --arg filter "$Filter"), have jq do the parsing, and then use getpath.
It would of course be simplest if GetSubobject could be called with the path separated from the field of interest, like this:
GetSubobject .b.x y filename