ID lookup from an external file in JQ - json

Question

I have a lookup file that maps IDs from one system onto another:
[
{
"idA": 2547,
"idB": "5d0bf91d191c6554d14572a6"
},
{
"idA": 2549,
"idB": "5b0473f93d4e53db19f8c249"
},
{
"idA": 2550,
"idB": "5d0bfabc8f20917b92ff07dc"
},
...
And I have a data file with values and an ID from one of these systems:
[
{
"idB": "5d0bf91d191c6554d14572a6",
"description": "Description for 5d0bf91d191c6554d14572a6"
},
{
"idB": "5d0bf49e9236c57281811cfc",
"description": "Description for 5d0bf49e9236c57281811cfc"
},
{
"idB": "5d0bfabc8f20917b92ff07dc",
"description": "Description for 5d0bfabc8f20917b92ff07dc"
},
...
I want to produce a new file of the descriptions with their IDs converted to the idA values in the lookup file. I tried this:
jq --slurpfile idmap ids.json 'map( {"description":.description, "id": (.idB as $b|$idmap[][]|select(.idB==$b)|.idA) } )' descriptions.json
But it produces only an empty array.
I have to double-dereference $idmap because slurping a file "binds an array of the parsed JSON values to the given global variable" -- so just doing $idmap[] throws an error, jq: error (at descriptions.json:70): Cannot index array with string "idB".
Can anyone explain what I'm doing wrong here?

Answer 1

Here's a concise and straightforward solution to the stated problem.
For simplicity, we'll begin by constructing a dictionary containing the relevant mapping using INDEX/2:
INDEX($idmap[]; .idB) | map_values(.idA)
Now the task is easy:
(INDEX($idmap[]; .idB) | map_values(.idA)) as $dict
| map( {description, "idA": $dict[.idB] } )
This assumes an invocation that uses --argfile idmap ids.json to avoid
the unwanted "slurping" caused by --slurpfile, but if the latter is used, then you would use $idmap[][] instead as noted in the original question.
Since the sample snippets do not include any matching "idB" values, there is little point in showing the output that would be obtained using these snippets.
Variation
If the objects in descriptions.js had other keys that should be retained, then the following variant would probably be a more useful guide:
(INDEX($idmap[]; .idB) | map_values(.idA)) as $dict # or $idmap[][] as above
| map( .idA = $dict[.idB] | del(.idB) )