从键值对中“过滤” JSON对象的最有效方法是什么？

Question

I am reading in a .json file. It's an array of objects in valid JSON format, example:

    [
        {
                "Id": 13,
                "Location": "Australia",
                "Content": "Another string"
        },
        {
                "Id": 145,
                "Location": "England",
                "Content": "SomeString"
        },
        {
                "Id": 12,
                "Location": "England",
                "Content": "SomeString"
        },
        {
                "Id": 12331,
                "Location": "Sweden",
                "Content": "SomeString"
        },
        {
                "Id": 213123,
                "Location": "England",
                "Content": "SomeString"
        }
     ]

I want to filter these objects out - say, removing anything where "Location"doesn't equal "England".

What I've tried so far is creating a custom UnmarshalJSON function. It does unmarshal it, but the objects it produces are empty - and as many as the input.

Sample code:

type languageStruct struct {
    ID                  int     `json:"Id"`
    Location            string  `json:"Location"` 
    Content             string  `json:"Content"`
}

func filterJSON(file []byte) ([]byte, error) {
    var x []*languageStruct

    err := json.Unmarshal(file, &x)
    check(err)

    return json.MarshalIndent(x, "", " ")
}


func (s *languageStruct) UnmarshalJSON(p []byte) error {

    var result struct {
        ID              int     `json:"Id"`
        Location        string  `json:"Location"` 
        Content         string  `json:"Content"`
    }

    err := json.Unmarshal(p, &result)
    check(err)

    // slice of locations we'd like to filter the objects on
    locations := []string{"England"} // Can be more 

    if sliceContains(s.Location, locations) {
        s.ID = result.ID
        s.Location= result.Location
        s.Content = result.Content
    }

    return nil
}

// helper func to check if a given string, f.e. a value of a key-value pair in a json object, is in a provided list
func sliceContains(a string, list []string) bool {
    for _, b := range list {
        if b == a {
            fmt.Println("it's a match!")
            return true
        }
    }
    return false
}

While this runs - the output is wrong. It creates as many objects as comes in - however, the new ones are empty, f.e.:

// ...
 [
 {
  "Id": 0,
  "Location": "",
  "Content": ""
 },
 {
  "Id": 0,
  "Location": "",
  "Content": ""
 }
 ]
//...

Whereas my desired output, from the first given input, would be:

[
    {
            "Id": 145,
            "Location": "England",
            "Content": "SomeString"
    },
    {
            "Id": 12,
            "Location": "England",
            "Content": "SomeString"
    },
    {
            "Id": 213123,
            "Location": "England",
            "Content": "SomeString"
    }
 ]