Updating Nested JSON Array with new key and value from another key - json

Question

I have have a JSON file where I have IDs with tasks. Some tasks can be empty. I want to put the ID into the tasks where tasks are not empty.
[
{
"id": 1961126,
"tasks": [
{
"id": 70340700,
"title": "Test1",
},
{
"id": 69801130,
"title": "Test15A",
}
]
},
{
"id": 1961126,
"tasks": []
}
]
I would like to get the tasks array updated to look like
[
{
"id": 1961126,
"tasks": [
{
**"sId":1961126,**
"id": 70340700,
"title": "Test1",
},
{
**"sId":1961126,**
"id": 69801130,
"title": "Test15A",
}
]
},
{
"id": 1961126,
"tasks": []
}
]
I can't figure out how to get the id from the object into the nested array. Here is what I have come up with
jq 'map(.tasks[0]|select( . != null )|.sId = .id)' file.json
This is only pulling in the same id. I have tired to put in [].id but I get a error Cannot index number with string "id". I am still learning how to deal with nested arrays and objects.

Answer 1

Save the ID in a variable and add it as a new field to each array member.
jq 'map(.id as $sId | .tasks[] += {$sId})' file.json
Demo
Note #1: Get rid of the final , within each object (see the Demo), as it's not proper JSON.
Note #2: Object fields generally have no order, but if you want to have the propagated ID shown first, as seen in your expected output, you could try to replace += {$sId} (which by itself is shorthand for |= . + {$sId}) with |= {$sId} + . to flip the order of generation (Demo). Although there is no guarantee that it stays that way with further processing.