如何计算 JSON 数组中按 JSONPath 对象属性过滤的对象

Question

我正在编写一个 JSON路径表达式，该表达式计算数组中的对象，同时按对象的字段之一进行过滤。

输入 JSON 来自 http://jsonpath.herokuapp.com/

{
    "store": {
        "book": [
            {
                "category": "reference",
                "author": "Nigel Rees",
                "title": "Sayings of the Century",
                "price": 8.95
            },
            {
                "category": "fiction",
                "author": "Evelyn Waugh",
                "title": "Sword of Honour",
                "price": 12.99
            },
            {
                "category": "fiction",
                "author": "Herman Melville",
                "title": "Moby Dick",
                "isbn": "0-553-21311-3",
                "price": 8.99
            },
            {
                "category": "fiction",
                "author": "J. R. R. Tolkien",
                "title": "The Lord of the Rings",
                "isbn": "0-395-19395-8",
                "price": 22.99
            }
        ],
        "bicycle": {
            "color": "red",
            "price": 19.95
        }
    },
    "expensive": 10
}

而我提取 "category" 对象的表达式是 "fiction"。

$.store.book[?(@.category == "fiction")]

输出符合预期。

[
   {
      "category" : "fiction",
      "author" : "Evelyn Waugh",
      "title" : "Sword of Honour",
      "price" : 12.99
   },
   {
      "category" : "fiction",
      "author" : "Herman Melville",
      "title" : "Moby Dick",
      "isbn" : "0-553-21311-3",
      "price" : 8.99
   },
   {
      "category" : "fiction",
      "author" : "J. R. R. Tolkien",
      "title" : "The Lord of the Rings",
      "isbn" : "0-395-19395-8",
      "price" : 22.99
   }
]

到目前为止一切顺利。我想要实现的是计算数组中对象的数量。在这种情况下 3. 我试过这个表达式：

$.store.book[?(@.category == "fiction")].length()

输出不是我预期的。

下面的也不行

$.store[?(@.book[*].category == "fiction")].length()

如何计算数组中元素的数量？如果我使用 jq 它可以写成

[.store.book[] | select (.category == "fiction")] | length

Answer 1

答案已经很晚了，但我最近遇到了这种情况，并根据问题中提供的 JSON 使用下面的 JsonPath 查询设法获取了数组内的对象数量：

$.length($.store.book[?(@.category == "fiction")].length())

如何计算 JSON 数组中按 JSONPath 对象属性过滤的对象

How to count JSON Objects in array filtered by attribute of the object by JSONPath

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