JOLT transformation to copy single value along an array - json

Question

I want to transform a JSON into key-value pairs and simultaneously copy a specific single value, i.e. the "timestamp", to all these pairs.
Input JSON:
{
"Timestamp": "2018-05-13T14:57:09",
"first_key": "1023",
"another_key": "1987",
"yet_another_key": "677"
}
Expected output:
[ {
"Timestamp": "2018-05-13T14:57:09.087",
"key": "first_key",
"value": "1023"
},
{
"Timestamp": "2018-05-13T14:57:09.087",
"key": "another_key",
"value": "1987"
},
{
"Timestamp": "2018-05-13T14:57:09.087",
"key": "yet_another_key",
"value": "677"
}]
What I came up with so far is the following JOLT specification. It already generates the key-value pairs for all entries that are not "Timestamp", but how can I copy the value of "Timestamp" into each of these records?
[{
"operation": "shift",
"spec": {
"Timestamp": "[].Timestamp",
"*": {
"$": "[#2].key",
"#": "[#2].value"
}
}
}]
Output of the above JOLT specification:
[ {
"Timestamp" : "2018-05-13T14:57:09"
}, {
"key" : "first_key",
"value" : "1023"
}, {
"key" : "another_key",
"value" : "1987"
}, {
"key" : "yet_another_key",
"value" : "677"
} ]

Answer 1

Spec
[
{
// first separate the Timestamp from the fields that
// are going to be pivoted.
// This is needed because the "[#2]" logic
// isn't doing what you think it is /
// you would end up with a null in your output array.
// Basically the "[#2]" logic as written only works
// when you are cleanly pivoting data. Which you are
// not because "Timestamp" and "first_key" are siblings.
"operation": "shift",
"spec": {
"Timestamp": "Timestamp",
"*": "keysToPivot.&"
}
},
{
"operation": "shift",
"spec": {
"keysToPivot": {
"*": {
// Now that we can cleanly loop thru _only_ the
// keysToPivot, the [#2] logic will work "right".
// Additionally, lookup the Timestamp value
// and add it to each "pivoted" output.
"$": "[#2].key",
"#": "[#2].value",
"#(2,Timestamp)": "[#2].Timestamp"
}
}
}
}
]