Leveling select fields - json

Question

I am fetching a json response of following structure:
{
"data": {
"children": [
{
"data": {
"id": "abcdef",
"preview": {
"images": [
{
"source": {
"url": "https://example.com/somefiles_1.jpg"
}
}
]
},
"title": "Boring Title One"
}
},
{
"data": {
"id": "ghijkl",
"preview": {
"images": [
{
"source": {
"url": "https://example.com/somefiles_2.jpg"
}
}
]
},
"title": "Boring Title Two"
}
},
{
"data": {
"id": "mnopqr",
"preview": {
"images": [
{
"source": {
"url": "https://example.com/somefiles_3.jpg"
}
}
]
},
"title": "Boring Title Three"
}
},
{
"data": {
"id": "stuvwx",
"preview": {
"images": [
{
"source": {
"url": "https://example.com/somefiles_4.jpg"
}
}
]
},
"title": "Boring Title Four"
}
}
]
}
}
Ideally I would like to have a shortened json like this:
{
"data": [
{
"id": "abcdef",
"title": "Boring Title One",
"url": "https://example.com/somefiles_1.jpg"
},
{
"id": "ghijkl",
"title": "Boring Title Two",
"url": "https://example.com/somefiles_2.jpg"
},
{
"id": "mnopqr",
"title": "Boring Title Three",
"url": "https://example.com/somefiles_3.jpg"
},
{
"id": "stuvwx",
"title": "Boring Title Four",
"url": "https://example.com/somefiles_4.jpg"
}
]
}
If this is not possible I can work with joining those three values into a single string and latter split when necessary; like this:
abcdef#Boring Title One#https://example.com/somefiles_1.jpg
ghijkl#Boring Title Two#https://example.com/somefiles_2.jpg
mnopqr#Boring Title Three#https://example.com/somefiles_3.jpg
stuvwx#Boring Title Four#https://example.com/somefiles_4.jpg
This is where I am. I was uring the jq with select() and then pipe the results to to_entries like this:
jq -r '.data.children[] | select(.data.post_type|test("image")?) | .data | to_entries[] | [ .value.title , .value.preview.images[0].source.url ] | join("#")' ~/Documents/json/sample.json
I don't understand what goes after to_entries[]; I have tried multiple variations of .key and .values; Mostly I don't get any result but sometimes I get key pairs I do not intend to select. How to learn the proper syntax for it?
Is creating a flat json out of a nested json like this good or is it better to create the string outputs? I feel the string might be error prone especially with the presence of spaces or special characters.

Answer 1

Apparently what you're looking for is the {field} syntax. You don't need to resort to string outputs.
{ data: [
.data.children[].data
| select(has("post_type") and (.post_type | index("image")))
| {id, title} + (.preview.images[].source | {url})
# or, if images array always contains one element:
# | {id, title, url: .preview.images[0].source.url}
]
}

Answer 2

A simple solution to the main question is:
{data: [.data.children[]
| .data
| {id, title, url: .preview.images[0].source.url} ]}
(The "post_type" seems to have disappeared, but hopefully if it's relevant, you will be able to adapt the above as required. Likewise if .images[1] and beyond are relevant.)
String Output
If you want linear output, you should probably consider CSV or TSV, both of which are supported by jq via #csv and #tsv respectively.