Get value of JSON object using jq --stream - json

Question

I'm trying to extract the value of an JSON object using jq --stream, because the real data can the size of multiple GigaBytes.
This is the JSON I'm using for my tests, where I want to extract the value of item:
{
"other": "content here",
"item": {
"A": {
"B": "C"
}
},
"test": "test"
}
The jq options I'm using:
jq --stream --null-input 'fromstream(inputs | select(.[0][0] == "item"))[]' example.json
However, I don't get any output with this command.
A strange thing I found is that when removing the object after the item the above command seems to work:
{
"other": "content here",
"item": {
"A": {
"B": "C"
}
}
}
The result looks as expected:
❯ jq --stream --null-input 'fromstream(inputs | select(.[0][0] == "item"))[]' example.json
{
"A": {
"B": "C"
}
}
But as I cannot control the input JSON this is not the solution.
I'm using jq version 1.6 on MacOS.

Answer 1

You didn't truncate the stream, therefore after filtering it to only include the parts below .item, fromstream is missing the final back-tracking item [["item"]]. Either add it manually at the end (not recommended, this would also include the top-level object in the result), or, much simpler, use 1 | truncate_stream to strip the first level altogether:
jq --stream --null-input '
fromstream(1 | truncate_stream(inputs | select(.[0][0] == "item")))
' example.json
{
"A": {
"B": "C"
}
}
Alternatively, you can use reduce and setpath to build up the result object yourself:
jq --stream --null-input '
reduce inputs as $in (null;
if $in | .[0][0] == "item" and has(1) then setpath($in[0];$in[1]) else . end
)
' example.json
{
"item": {
"A": {
"B": "C"
}
}
}
To remove the top level object, either filter for .item at the end, or, similarly to truncate_stream, remove the path's first item using [1:] to strip the first level:
jq --stream --null-input '
reduce inputs as $in (null;
if $in | .[0][0] == "item" and has(1) then setpath($in[0][1:];$in[1]) else . end
)
' example.json
{
"A": {
"B": "C"
}
}