jq: where select clause is applied

Here my documents:

[
   {
      "id":"f3b8c257-9950-45e7-9e79-ace19ec8905e",
      "identifier":[
         {
            "system":{
               "value":"urn:oid:2.16.724.4.9.10.2"
            },
            "value":{
               "value":"10839812"
            }
         }
      ]
   },
   {
      "id":"f0a1e3ae-826f-4a03-b29e-10ef3bc86ea0",
      "identifier":null
   }
]

Currently, I’m aplying this jq filter:

map(
    {
        id,
        dni: .identifier[] | (select(.system.value == "urn:oid:1.3.6.1.4.1.19126.3") | .value.value)
    }
)

However, I’m getting this message:

jq: error (at practitioner-mongoexport.json:146715): Cannot iterate over null (null)

As you can guess, problem arises processing second object where .identifier: null.

I’ve tried with that:

map(
    {
        id,
        dni: select(.identifier) | .identifier[] | (select(.system.value == "urn:oid:1.3.6.1.4.1.19126.3") | .value.value)
    }
)

I’ve also tried with:

map(
    select(.identifier) |
    {
        id,
        dni: .identifier[] | (select(.system.value == "urn:oid:1.3.6.1.4.1.19126.3") | .value.value)
    }
)

Then result is [].

I don’t quite figure out what I’m doing wrong.

My desired output would be:

[
   {
      "id":"f3b8c257-9950-45e7-9e79-ace19ec8905e",
      "dni": "10839812"
   }
]

Any ideas?

>Solution :

map(
    select(.identifier?[]?.system.value == "urn:oid:2.16.724.4.9.10.2") 
    | { id, "dni": .identifier[].value.value }
)

Will generate:

[
  {
    "id": "f3b8c257-9950-45e7-9e79-ace19ec8905e",
    "dni": "10839812"
  }
]

The trick here is .identifier?[]?.system.value where the ? will ignore .identifier if it’s null

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