jq transform JSON child array objects to delimited string - json

Question

I want to transform the following input with jq:
{
"root":[
{
"field1":"field1value1",
"field2":"field2value2",
"field3Array":[
{
"prop1":"prop1_value1"
}
]
},
{
"field1":"field1value3",
"field2":"field2value4",
"field3Array":[
{
"prop1":"prop1_value3"
},
{
"prop1":"prop1_value4"
}
]
}
]
}
Output should be:
[
{
"field1": "field1value1",
"field2": "field2value2",
"field3Array": "prop1_value1"
},
{
"field1": "field1value3",
"field2": "field2value4",
"field3Array": "prop1_value3,prop1_value4"
}
]
I use this jq filter so far:
[.root[] | {field1, field2, field3Array: .field3Array[].prop1}]
but I don't know how to join the array property "prop1" to a comma-delimited string "prop1_value3,prop1_value4".
https://jqplay.org/s/CR8mGBX8Dz

Answer 1

You need to map the objects contained in the field3Array to their string values and join the resulting array :
.root | map({field1, field2, field3Array: .field3Array | map(.prop1) | join(",")})
You can try it here.
It can be somewhat simplified in the following where we update the .field3Array in-place instead of recreating a whole object :
.root | map(.field3Array |= (map(.prop1) | join(",")))
You can try it here.
If you're unfamiliar with the map function, the following would have worked as well :
[.root[] | {field1, field2, field3Array: [ .field3Array[] | .prop1 ] | join(",")}]
You can try it here.