merge two lists within a object conditionally - json

Question

Hellow jq experts!
I'm a jq learner and have a json obect composed lists as follows:
{
"image_files": [
{
"id": "img_0001",
"width": 32,
"heigt": 32,
"file_name": "img_0001.png"
},
{
"id": "img_0002",
"width": 128,
"heigt": 32,
"file_name": "img_0002.png"
},
{
"id": "img_0003",
"width": 32,
"heigt": 32,
"file_name": "img_0003.png"
},
{
"id": "img_0004",
"width": 160,
"heigt": 32,
"file_name": "img_0004.png"
}
],
"annotations": [
{
"id": "ann_0001",
"image_id": "img_0001",
"label": "A",
"attributes": {
"type": "letter",
"augmented": false
}
},
{
"id": "ann_0002",
"image_id": "img_0002",
"label": "Good",
"attributes": {
"type": "word",
"augmented": false
}
},
{
"id": "ann_0003",
"image_id": "img_0003",
"label": "C",
"attributes": {
"type": "letter",
"augmented": false
}
},
{
"id": "ann_0004",
"image_id": "img_0004",
"label": "Hello",
"attributes": {
"type": "word",
"augmented": false
}
}
]
}
image_id in the annotations list are foreign key referencing the id in the image_files list.
I want to join image_files and annotations with condition of annotations.attribute.type == "letter".
Expecting following ouptut:
{
"letter_image_files_with_label": [
{
"id": "img_0001",
"width": 32,
"heigt": 32,
"file_name": "img_0001.png",
"label": "A"
},
{
"id": "img_0003",
"width": 32,
"heigt": 32,
"file_name": "img_0003.png",
"label": "C"
}
]
}
How can I produce above result from the json data input?
join explained in jq manual does not seem to use this kind task.
Is there a way for this? Please show me the rope.
Thanks for your generous reading.

Answer 1

Indexing image_files with ids makes this pretty trivial.
INDEX(.image_files[]; .id) as $imgs | [
.annotations[]
| select(.attributes.type == "letter")
| $imgs[.image_id] + {label: .label}
]
Online demo