Flatten a JSON document using jq

This one was a tricky one to craft.

map
(
    with_entries(select(.key != "fields"))
    +
    (.fields | with_entries(.value = .value[0]))
)

Let's break it down and explain the bits of it

1. For every item in the array...

   map(...)
   

2. Create a new object containing the values for all except the fields property.

   with_entries(select(.key != "fields"))
   

3. Combine that with...

   +
   

4. Each of the fields projecting each of the values to the first item of each array

   (.fields | with_entries(.value = .value[0]))
   

There's a tool called gron and you can pipe that json in, to get something like this.

$ gron document.json
json = [];
json[0] = {};
json[0].fields = {};
json[0].fields.country = [];
json[0].fields.country[0] = "DE";
json[0].fields.deviceID = [];
json[0].fields.deviceID[0] = "deviceID1";
json[0].fields.deviceOs = [];
json[0].fields.deviceOs[0] = "Android";
json[0].fields.type = [];
json[0].fields.type[0] = "type";
json[0].id = "id1";
json[0].index = "index1";
json[0].type = "type1";
json[1] = {};
json[1].fields = {};
json[1].fields.country = [];
json[1].fields.country[0] = "US";
json[1].fields.deviceID = [];
json[1].fields.deviceID[0] = "deviceID2";
json[1].fields.deviceOs = [];
json[1].fields.deviceOs[0] = "Android";
json[1].fields.type = [];
json[1].fields.type[0] = "type";
json[1].id = "id2";
json[1].index = "index2";
json[1].type = "type2";