jq: Cannot iterate over number/string and number cannot be added

In my continued parsing of meetup.com’s JSON API I wanted to extract some information from the following JSON file:

$ head -n40 data/members/18313232.json
[
  {
    "status": "active",
    "city": "London",
    "name": ". .",
    "other_services": {},
    "country": "gb",
    "topics": [],
    "lon": -0.13,
    "joined": 1438866605000,
    "id": 92951932,
    "state": "17",
    "link": "http://www.meetup.com/members/92951932",
    "photo": {
      "thumb_link": "http://photos1.meetupstatic.com/photos/member/8/d/6/b/thumb_250896203.jpeg",
      "photo_id": 250896203,
      "highres_link": "http://photos1.meetupstatic.com/photos/member/8/d/6/b/highres_250896203.jpeg",
      "photo_link": "http://photos1.meetupstatic.com/photos/member/8/d/6/b/member_250896203.jpeg"
    },
    "lat": 51.49,
    "visited": 1446745707000,
    "self": {
      "common": {}
    }
  },
  {
    "status": "active",
    "city": "London",
    "name": "Abdelkader Idryssy",
    "other_services": {},
    "country": "gb",
    "topics": [
      {
        "name": "Weekend Adventures",
        "urlkey": "weekend-adventures",
        "id": 16438
      },
      {
        "name": "Community Building",
        "urlkey": "community-building",

In particular I want to extract the member’s id, name, join date and the ids of topics they’re interested in. I started with the following jq query to try and extract those attributes:

$ jq -r '.[] | [.id, .name, .joined, (.topics[] | .id | join(";"))] | @csv' data/members/18313232.json
Cannot iterate over number (16438)

Annoyingly this treats topic ids on an individual basis rather than as an array as I wanted. I tweaked the query to the following with no luck:

$ jq -r '.[] | [.id, .name, .joined, (.topics[].id | join(";"))] | @csv' data/members/18313232.json
Cannot iterate over number (16438)

As a guess I decided to wrap ‘.topics[].id’ in an array literal to see if it had any impact:

$ jq -r '.[] | [.id, .name, .joined, ([.topics[].id] | join(";"))] | @csv' data/members/18313232.json
92951932,". .",1438866605000,""
jq: error (at data/members/18313232.json:60013): string ("") and number (16438) cannot be added

Woot! A different error message at least and this one seems to be due to a type mismatch between the string we want to end up with and the array of numbers that we currently have.

We can cast our way to victory with the ‘tostring’ function:

$ jq -r '.[] | [.id, .name, .joined, ([.topics[].id | tostring] | join(";"))] | @csv' data/members/18313232.json
...
92951932,". .",1438866605000,""
193866304,"Abdelkader Idryssy",1445195325000,"16438;20727;15401;9760;20246;20923;3336;2767;242;58259;4417;1789;10454;20274;10232;563;25375;16433;15187;17635;26273;21808;933;7789;23884;16212;144477;15322;21067;3833;108403;20221;1201;182;15083;9696;4377;15360;18296;15121;17703;10161;1322;3880;18333;3485;15585;44584;18692;21681"
28643052,"Abhishek Chanda",1439688955000,"646052;520302;15167;563;65735;537492;646072;537502;24959;1025832;8599;31197;24410;26118;10579;1064;189;48471;16216;18062;33089;107633;46831;20479;1423042;86258;21441;3833;21681;188;9696;58162;20398;113032;18060;29971;55324;30928;15261;58259;638;16475;27591;10107;242;109595;10470;26384;72514;1461192"
39523062,"Adam Kinder-Jones",1438677474000,"70576;21549;3833;42277;164111;21522;93380;48471;15167;189;563;25435;87614;9696;18062;58162;10579;21681;19882;108403;128595;15582;7029"
194119823,"Adam Lewis",1444867994000,"10209"
14847001,"Adam Rogers",1422917313000,""
87709042,"Adele Green",1436867381000,"15167;18062;102811;9696;30928;18060;78565;189;7029;48471;127567;10579;58162;563;3833;16216;21441;37708;209821;15401;59325;31792;21836;21900;984862;15720;17703;96823;4422;85951;87614;37428;2260;827;121802;19672;38660;84325;118991;135612;10464;1454542;17936;21549;21520;17628;148303;20398;66339;29661"
11497937,"Adrian Bridgett",1421067940000,"30372;15046;25375;638;498;933;374;27591;18062;18060;15167;10581;16438;15672;1998;1273;713;26333;15099;15117;4422;15892;242;142180;563;31197;20479;1502;131178;15018;43256;58259;1201;7319;15940;223;8652;66493;15029;18528;23274;9696;128595;21681;17558;50709;113737"
14151190,"adrian lawrence",1437142198000,"7029;78565;659;85951;15582;48471;9696;128595;563;10579;3833;101960;16137;1973;78566;206;223;21441;16216;108403;21681;186;1998;15731;17703;15043;16613;17885;53531;48375;16615;19646;62326;49954;933;22268;19243;37381;102811;30928;455;10358;73511;127567;106382;16573;36229;781;23981;1954"
183557824,"Adrien Pujol",1421882756000,"108403;563;9696;21681;188;24410;1064;32743;124668;15472;21123;1486432;1500742;87614;46831;1454542;46810;166000;126177;110474"
...