How Can We Modify Nested Struct Fields Using Reflection to Achieve Persistence?

Using Reflection to Set Nested Struct Fields

Problem:

How can we use reflection to modify the value of a nested struct field in a way that persists?

Code:

<code class="go">type ProductionInfo struct {
    StructA []Entry
}

type Entry struct {
    Field1 string
    Field2 int
}

func SetField(source interface{}, fieldName string, fieldValue string) {
    v := reflect.ValueOf(source)
    tt := reflect.TypeOf(source)

    for k := 0; k < tt.NumField(); k++ {
        fieldValue := reflect.ValueOf(v.Field(k))

        fmt.Println(fieldValue.CanSet())
        if fieldValue.CanSet() {
            fieldValue.SetString(fieldValue.String())
        }
    }
}

func main() {
    source := ProductionInfo{}
    source.StructA = append(source.StructA, Entry{Field1: "A", Field2: 2})

    SetField(source, "Field1", "NEW_VALUE")
}</code>

Issues:

1. Passing the wrong value: We're attempting to set the field of the top-level ProductionInfo struct, instead of the nested Entry struct. This causes errors because ProductionInfo does not have a Field1 field.
2. Passing a non-pointer: Reflection cannot modify the fields of non-pointer structs. We need to pass a pointer to the Entry struct.

Solution:

1. Modify the call to SetField to target the embedded Entry struct:

   <code class="go">SetField(source.StructA[0], "Field1", "NEW_VALUE")</code>

2. Change the function to accept and modify a pointer to the Entry:

   <code class="go">func SetField(source *Entry, fieldName string, fieldValue string) {
    v := reflect.ValueOf(source).Elem()
   
    fmt.Println(v.FieldByName(fieldName).CanSet())
   
    if v.FieldByName(fieldName).CanSet() {
        v.FieldByName(fieldName).SetString(fieldValue)
    }
   }</code>

Final Code:

<code class="go">package main

import (
    "fmt"
    "reflect"
)

type ProductionInfo struct {
    StructA []Entry
}

type Entry struct {
    Field1 string
    Field2 int
}

func SetField(source *Entry, fieldName string, fieldValue string) {
    v := reflect.ValueOf(source).Elem()

    if v.FieldByName(fieldName).CanSet() {
        v.FieldByName(fieldName).SetString(fieldValue)
    }
}

func main() {
    source := ProductionInfo{}
    source.StructA = append(source.StructA, Entry{Field1: "A", Field2: 2})

    fmt.Println("Before: ", source.StructA[0])
    SetField(&source.StructA[0], "Field1", "NEW_VALUE")
    fmt.Println("After: ", source.StructA[0])
}</code>

Output:

Before:  {A 2}
true
After:  {NEW_VALUE 2}

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