Use jq to combine two arrays of objects on a certain key - jq

Question

I am trying to use jq to solve this problem.
Suppose I have the following object
{
"listA": [
{
"id": "12345",
"code": "001"
}
]
"listB": [
{
"id": "12345",
"prop": "AABBCC"
}
]
}
In reality my two lists are longer, but the id isn't repeated within each list.
How may I combine the two lists into a single list where each item is an object with the non-id properties for the given id are collected into a single object?
For example, from the object above, I'd like the following:
{
"listC" : [
{
"id": "12345",
"code": "001",
"prop": "AABBCC"
}
]
}

Answer 1

A simple way would be to concatenate the arrays, group the elements by id and map each group into a single object using add;
jq '.listA+.listB | group_by(.id) | map(add)' test.json
If there may be more than two arrays you need to merge in the file, you could instead use flatten to concatenate all of them.
Test case below
# cat test.json
{
"listA": [
{ "id": "12345", "code": "001" },
{ "id": "12346", "code": "002" }
],
"listB": [
{ "id": "12345", "prop": "AABBCC" }
]
}
# jq 'flatten | group_by(.id) | map(add)' test.json
# or
# jq '.listA+.listB | group_by(.id) | map(add)' test.json
[
{
"id": "12345",
"code": "001",
"prop": "AABBCC"
},
{
"id": "12346",
"code": "002"
}
]

Answer 2

Using group_by entails a sort, which is unnecessary, so if efficiency is a concern, then an alternative approach such as the following should be considered:
INDEX(.listA[]; .id) as $one
| INDEX(.listB[]; .id) as $two
| reduce ($one|keys_unsorted[]) as $k ($two; .[$k] += $one[$k])
| {listC: [.[]] }