json - 如果嵌套数组为空，jq 将不返回任何内容

Question

这里填充了数组 subThings

$ echo '[{"thing-1" : "meta1","thing-2" : "meta2","thing-n" : "metan","subThings":[{"subThing-1" : "subMeta1","subThing-2" : "subMeta2","subThing-n" : "subMetan"}]}]' | jq '.'
[
  {
    "subThings": [
      {
        "subThing-n": "subMetan",
        "subThing-2": "subMeta2",
        "subThing-1": "subMeta1"
      }
    ],
    "thing-n": "metan",
    "thing-2": "meta2",
    "thing-1": "meta1"
  }
]

您可以在此处看到缺少的键仅设置为 null。

$ echo '[{"thing-1" : "meta1","thing-2" : "meta2","thing-n" : "metan","subThings":[{"subThing-1" : "subMeta1","subThing-2" : "subMeta2","subThing-n" : "subMetan"}]}]' |
 jq '[.[] | {"thing-1","thing-5","subThing-2":.subThings[]["subThing-2"],}]'
[
  {
    "subThing-2": "subMeta2",
    "thing-5": null,
    "thing-1": "meta1"
  }
]

此处数组 subThings 为空

$ echo '[{"thing-1" : "meta1","thing-2" : "meta2","thing-n" : "metan","subThings":[]}]' | jq '.'
[
  {
    "subThings": [],
    "thing-n": "metan",
    "thing-2": "meta2",
    "thing-1": "meta1"
  }
]

它会清除所有数据，而不是 subThing-2 的 null。

$ echo '[{"thing-1" : "meta1","thing-2" : "meta2","thing-n" : "metan","subThings":[]}]' |
  jq '[.[] | {"thing-1","thing-5","subThing-2":.subThings[]["subThing-2"],}]'
[]
$ 

为什么？
有没有不同的方法来获取数据而不用空数组清除所有内容？

Answer 1

您正在尝试根据 subThings 对象的存在来生成对象。这实际上就是 .subThings[] 在 {"thing-1","thing-5","subThing-2":.subThings[]["subThing-2"]} 上下文中所做的事情。

您需要单独评估 subThings ，如果没有，则替换为 null ，否则它将只是 empty ，不会产生任何结果(这就是您在这里看到的)。

map({ "thing-1", "thing-5", "subThing-2": (.subThings[]."subThing-2" // null) })