Merge json files on a "primary key" using jq

October 12, 2022

I have two files containing json arrays like so

file1.json

[
  {
    "classid":"abc",
    "name":"Alex"
  },
  {
    "classid":"abc",
    "name":"Bob"
  }
]

file2.json

[
  {
    "classid":"abc",
    "classname":"math"
  }
]

I’d like to combine these two files on "classid".
Desired output.json

[
  {
    "classid":"abc",
    "name":"Alex",
    "classname":"math
  },
  {
    "classid":"abc",
    "name":"Bob",
    "classname":"math
  }
]

I tried
> jq -s 'reduce .[] as $item ({}; . *= $item)' file1.json file2.json
but I see a jq: error (at file1.json:10): object ({}) and array ([{"classid...) cannot be multiplied

Would appreciate any help.

>Solution :

If you have jq 1.6, just use JOIN with an INDEX:

jq '[JOIN(INDEX(input[]; .classid); .[]; .classid; add)]' file1.json file2.json

[
  {
    "classid": "abc",
    "name": "Alex",
    "classname": "math"
  },
  {
    "classid": "abc",
    "name": "Bob",
    "classname": "math"
  }
]

Demo