Interaction between {} and select - jq

Question

Here's my test data:
[
{
"id": "id-1",
"tags": {
"key": "name",
"value": "name-1"
}
},
{
"id": "id-2"
}
]
I'm trying to simplify the output, to show the 'name' field if present, and always show the id. For example, this script almost works:
~ $ cat testdata | jq '.[] | {id, name: .tags.value}'
{
"id": "id-1",
"name": "name-1"
}
{
"id": "id-2",
"name": null
}
When I try to add in a guard against .keys not existing and filter for the section of 'keys' I care about, here's what happens:
~ $ cat testdata | jq '.[] | {id, name: (select(.tags.key == "name") | .tags.value)}'
{
"id": "id-1",
"name": "name-1"
}
I assume {} is somehow ending up with a zero-length array instead of 'null'. What should I be using instead of |? What am I misunderstanding?

Answer 1

I ended up solving the problem using: [POSSIBLY_MATCHED_EXPRESSION][0], in this case:
cat testdata | jq '.[] | {id, name: ([select(.tags.key == "name") | .tags.value][0])}'

Answer 2

If I'm understanding correctly, if you wanted to include a name only if it existed, I'd do this:
map({id} + with_entries(select(.key == "tags") | .value))
Otherwise if you don't mind null names:
map({id, name: with_entries(select(.key == "tags") | .value) | .name})
Here's a more general solution if you have other "tags" so it's not hardcoded to only accept name values.
This assumes that any object value is actually a key/value pair.
map(with_entries(if .value | type == "object" then .value else . end))
Or if tags is the only dynamic property:
map(with_entries(if .key == "tags" then .value else . end))

Answer 3

If the goal is to produce:
{"id":"id-1","name":"name-1"}
{"id":"id-2"}
then the following three expressions are essentially equivalent solutions:
.[] | if .tags.key == "name" then {id, name: .tags.value} else {id} end
.[] | {id} + (if .tags.key == "name" then {name: .tags.value} else {} end)
.[] | (select(.tags.key == "name") | {id, name: .tags.value}) // {id}

Answer 4

You could just add
| if .name == null then del(.name) else . end
to the end of your filter to get rid of the .name key when its value is null.
With your test data, the following
.[]
| {id, name:.tags.value }
| if .name == null then del(.name) else . end
produces
{
"id": "id-1",
"name": "name-1"
}
{
"id": "id-2"
}