jq：选择而不从数组中删除项目

Question

I have a JSON file which looks like this (a rough schema):

[{
    "custom_variables": [
        {
            "name": "xxx",
            "value": "xxx"
        },
        {
            "name": "xxx",
            "value": "xxx"
        },
        {
            "name": "profile_id",
            "value": "123"
        }
    ],
    // many fields
    "xxx": "xxx",
    "xxx": "xxx",
    "xxx": "xxx"
}]

I am using jq to extract all the fields from the top level object. The custom_variables field constains an array of objects with name and value.

I want to extract a specific object from custom_variables, given its name.

So what I am doing is this:

jq 'map(
    {
        xxx: .xxx, 
        xxx: .xxx, 
        xxx: .xxx, 
        xxx: .custom_variables | .[] | select(.name == "variable_name")
    } 
)'

It almost works; it gets the variable I want when it exists, but when it doesn't (or if custom_variables itself doesn't) it will drop the whole top-level object. So in the end I am getting less objects then I put into the script.

How can I just return null if I don't find that field but still keep the rest of the data?

Answer 1

Use the alternative operator ( // ) to transform a stream of zero elements (such as those which can be generated by select or .[] ) into a value:

jq 'map(
    {
        xxx: .xxx, 
        xxx: .xxx, 
        xxx: .xxx, 
        xxx: .custom_variables | .[] | select(.name == "variable_name") // null
    } 
)'

This will have .xxx be null when there is a zero-element stream on the left side of // .

You can, of course, place the alternative operator at a different position to capture the zero-element stream at an earlier or later stage, such as at the object level:

jq 'map(
    {
        xxx: .xxx, 
        xxx: .xxx, 
        xxx: .xxx, 
        xxx: .custom_variables | .[] | select(.name == "variable_name")
    } // {}
)'

Answer 2

This works, but it looks ugly. Any better solutions?

custom_variables: (if (.custom_variables | length > 0) 
            then (.custom_variables | .[]? | select(.name == "variable_name") | .value | scan("\\d+"))
            else null
            end)

Answer 3

The following does what you've indicated you want, as best as I can understand your requirements.

map( if .custom_variables
     then .custom_variables |= (map(select(.name == "variable_name") | .value)
                                | .[0]) 
     else .
     end )

Example input:

[{
    "custom_variables": [
        {
            "name": "xxx",
            "value": "xxx"
        },
        {
            "name": "xxx",
            "value": "xxx"
        },
        {
            "name": "variable_name",
            "value": "123"
        }
    ],
    "xxx1": "xxx",
    "xxx2": "xxx",
    "xxx3": "xxx"
},

{
    "yyy1": "yyy",
    "yyy2": "yyy",
    "yyy3": "yyy"
}
]

Output:

[
  {
    "custom_variables": "123",
    "xxx1": "xxx",
    "xxx2": "xxx",
    "xxx3": "xxx"
  },
  {
    "yyy1": "yyy",
    "yyy2": "yyy",
    "yyy3": "yyy"
  }
]