How to Dynamically Control JSON Response Fields in Go?

Dynamically Removing or Hiding Fields in JSON Responses

When working with API responses, it's often useful to control the specific fields that are returned to the caller. In Go, structs are commonly used to represent data that is encoded into JSON. However, statically-defined struct tags may not provide enough flexibility to dynamically remove or hide specific fields.

Removing Fields from a Struct

It's not possible to dynamically remove fields from a struct in Go. Fields that are declared in a struct are permanently part of the type definition.

Hiding Fields in JSON Response

The JSON omitempty tag can be used to hide empty fields in a JSON response. However, this approach is not suitable for situations where you need to hide non-empty fields.

Using Maps Instead of Structs

One way to dynamically control the fields included in the response is to use a map[string]interface{}. Maps are unordered collections of key-value pairs. You can remove fields from a map using the delete built-in function.

package main

import (
    "encoding/json"
    "fmt"
)

type SearchResults struct {
    NumberResults int                `json:"numberResults"`
    Results       []map[string]interface{} `json:"results"`
}

func main() {
    // Assume we obtained the following map from a query
    result := map[string]interface{}{
        "idCompany":   1,
        "company":     "Acme Inc.",
        "industry":    "Manufacturing",
        "idState":     5,
        "state":       "New York",
        "country":     "US",
        "otherField1": "Some Value 1",
        "otherField2": 2.3,
    }

    // Create a SearchResults struct
    searchResults := SearchResults{
        NumberResults: 1,
        Results:       []map[string]interface{}{result},
    }

    // Remove any fields not specified in the `fields` GET parameter
    fields := []string{"idCompany", "company", "state"}
    for k, v := range searchResults.Results {
        for f := range v {
            if !contains(fields, f) {
                delete(v, f)
            }
        }
    }

    // Encode the modified SearchResults as JSON
    jsonBytes, _ := json.Marshal(searchResults)

    // Print the JSON
    fmt.Println(string(jsonBytes))
}

func contains(s []string, e string) bool {
    for _, a := range s {
        if a == e {
            return true
        }
    }
    return false
}

In this example, the fields to be returned are specified in the fields GET parameter. The code iterates over the map, removing any fields that are not included in the specified list. Finally, the modified map is encoded as JSON and returned to the caller.

Alternative Approach

Another alternative is to query the database only for the requested fields. This approach requires modifying the SQL query to include only the desired fields. While this method is more efficient, it may not be feasible in all cases.

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