Retrieve one key value from another file at another depth - jq

Question

I have two json files (these are from AWS). One is returned from amazon following a server state change (state.json), the other has details of the instance including a specific tag (The tag is called "Name" and the value has the hostname of the server) - file is called instance.json in my example. I'm trying to write some jq which uses the instanceID retrieved from the state to query the instance details document (with the same instanceID key). I think if I could get the hostname tag added to the state document in the right place, that would be ideal...
This would normally be something I'd possibly be capable of, but the keys are at different depths in the json and I can't figure out how to retrieve different depth/matching keys. (If you're familiar with aws you'll know the server state is also in the instance, however I'm changing the state and don't wish to make 3 amazon calls).
Some sample json below:
instance.json (this is a huge file, I've edited out all the useless bits and maintained the structure):
{
"Reservations": [
{
"Instances": [
{
"InstanceId": "i-1",
"Tags": [
{
"Value": "hostname1",
"Key": "Name"
}
],
"AmiLaunchIndex": 0
}
],
"ReservationId": "r-1",
"Groups": []
},
{
"Instances": [
{
"InstanceId": "i-2",
"Tags": [
{
"Value": "hostname2",
"Key": "Name"
}
],
"AmiLaunchIndex": 0
}
],
"ReservationId": "r-1",
"Groups": []
},
{
"Instances": [
{
"InstanceId": "i-3",
"Tags": [
{
"Value": "hostname3",
"Key": "Name"
}
],
"AmiLaunchIndex": 0
}
],
"ReservationId": "r-1",
"Groups": []
}
]
}
state.json:
{
"StoppingInstances": [
{
"CurrentState": {
"Code": 80,
"Name": "stopped"
},
"InstanceId": "i-1",
"PreviousState": {
"Code": 80,
"Name": "stopped"
}
},
{
"CurrentState": {
"Code": 80,
"Name": "stopped"
},
"InstanceId": "i-2",
"PreviousState": {
"Code": 80,
"Name": "stopped"
}
},
{
"CurrentState": {
"Code": 80,
"Name": "stopped"
},
"InstanceId": "i-3",
"PreviousState": {
"Code": 80,
"Name": "stopped"
}
}
]
}
Desirable output (if possible):
{
"StoppingInstances": [
{
"CurrentState": {
"Code": 80,
"Name": "stopped"
},
"InstanceId": "i-1",
"Hostname": "hostname1",
"PreviousState": {
"Code": 80,
"Name": "stopped"
}
},
{
"CurrentState": {
"Code": 80,
"Name": "stopped"
},
"InstanceId": "i-2",
"Hostname": "hostname2",
"PreviousState": {
"Code": 80,
"Name": "stopped"
}
},
{
"CurrentState": {
"Code": 80,
"Name": "stopped"
},
"InstanceId": "i-3",
"Hostname": "hostname3",
"PreviousState": {
"Code": 80,
"Name": "stopped"
}
}
]
}

Answer 1

Here's a straightforward approach that just uses INDEX. It assumes an invocation of the following form, though different variations are of course possible:
jq -n --argfile state state.json --argfile instance instance.json -f program.jq
where program.jq contains:
INDEX($instance.Reservations[].Instances[]; .InstanceId)
| map_values(.Tags|from_entries.Name) as $dict
| $state
| .StoppingInstances |= map(. + {Hostname: $dict[.InstanceId]})
If you're not sure where .InstanceId is located in instance.json, you could modify the above as follows:
INDEX($instance | .. | objects | select(has("Instances")) | .Instances[];
.InstanceId)
| map_values(.Tags|from_entries.Name) as $dict
| $state
| .StoppingInstances |= map(. + {Hostname: $dict[.InstanceId]})

Answer 2

To query specific tags in instance.json based on ids shared with state.json, you can iterate over the targets by comparing their common ids using a variable and the select function.
jq -r --argfile state state.json --arg tag "Name" '
$state.StoppingInstances[].InstanceId as $id
| .Reservations[].Instances[]
| select(.InstanceId == $id).Tags[]
| select(.Key == $tag).Value
' instance.json
hostname1
hostname2
hostname3
To join records from instance.json with items in state.json, you could use the INDEX and JOIN builtins:
jq --argfile instance instance.json '
.StoppingInstances |= JOIN(
INDEX($instance.Reservations[].Instances[]; .InstanceId); .InstanceId
)
' state.json
{
"StoppingInstances": [
[
{"CurrentState":{"Code":80,"Name":"stopped"},"InstanceId":"i-1","PreviousState":{"Code":80,"Name":"stopped"}},
{"InstanceId":"i-1","Tags":[{"Value":"hostname1","Key":"Name"}],"AmiLaunchIndex":0}
],
[
{"CurrentState":{"Code":80,"Name":"stopped"},"InstanceId":"i-2","PreviousState":{"Code":80,"Name":"stopped"}},
{"InstanceId":"i-2","Tags":[{"Value":"hostname2","Key":"Name"}],"AmiLaunchIndex":0}
],
[
{"CurrentState":{"Code":80,"Name":"stopped"},"InstanceId":"i-3","PreviousState":{"Code":80,"Name":"stopped"}},
{"InstanceId":"i-3","Tags":[{"Value":"hostname3","Key":"Name"}],"AmiLaunchIndex":0}
]
]
}