Flatten JSON with jq retaining key names - json

Question

I'm trying to flatten a JSON consisting of nested objects. The top layer contains several key/value pairs, where each value is itself an array of a number of objects (the bottom layer).
What I would like to get, using jq, is simply an array of objects containing all the objects of the bottom layer, each of which with an additional key/value pair identifying the top-layer key it originally belonged to.
In other words, I would like to turn a JSON
{
"key1": [obj1, obj2],
"key2": [obj3]
}
into a plain array
[OBJ1, OBJ2, OBJ3]
where each OBJi is simply the original object with an extra key/value pair
"parent-key-name": keyx
where keyx would be the top-layer key obji belonged to, i.e. "key1" for obj1 and obj2, and "key2" for obj3.
I'm struggling with the fact that when referencing the objects in the bottom layer, e.g. via .[], jq does not seem to have inbuilt functionality to access associated top-layer information. However, I'm new to jq, and hope there is an easy solution after all.

Answer 1

Given the following input :
{
"key1": [{"name":"Emma"},{"name":"Bob"}],
"key2": [{"name":"Jean"}]
}
You can divide your items to entries, store the key in a variable and add the value for each item in value object:
jq '[ to_entries[] | .key as $parent | .value[] |
.["parent-key-name"] |= (.+ $parent) ] ' test.json
which gives the following output :
[
{
"name": "Emma",
"parent-key-name": "key1"
},
{
"name": "Bob",
"parent-key-name": "key1"
},
{
"name": "Jean",
"parent-key-name": "key2"
}
]

Answer 2

The solution presented below consists of two steps, each of which might be helpful separately, e.g. if someone wants to "flatten" the JSON in a slightly different way.
First, let's make the changes to obj[i] "in-place":
with_entries( .key as $k | .value[] |= ( . + {"parent-key-name": $k} ) )
Example:
$ jq -n -c -f program.jq
Input:
{
"key1": [{a:1}, {a:2}],
"key2": [{b:3}]
}
Output:
{
"key1": [
{
"a": 1,
"parent-key-name": "key1"
},
{
"a": 2,
"parent-key-name": "key1"
}
],
"key2": [
{
"b": 3,
"parent-key-name": "key2"
}
]
}
To flatten, simply append | [.[]] to the above filter. This produces:
[[{"a":1,"parent-key-name":"key1"},{"a":2,"parent-key-name":"key1"}],[{"b":3,"parent-key-name":"key2"}]]