Updating struct field using reflection

I'm implementing partial update for an entity.

The entity struct looks like

type Entity struct {
	Id       string `readonly:"true"`
	Spec     EntitySpec
	Status   EntityState
}


type EntitySpec struct {
	Version  *string `readonly:"true"`
	Users    []*User
	Field1   *InnerStruct1
	Field2   []*InnerStruct2
}

and so on.

So I'm trying to use reflection to iterate over Entity struct fields recursively and update fields which user is allowed to update:

func method(existingEntity interface{}, newEntity interface{}) {
	entityType := reflect.TypeOf(existingEntity)
	logger.Debugf("type: %v", entityType)
	for i := 0; i < entityType.NumField(); i++ {
		value := entityType.Field(i)
		logger.Debugf("Name: %s", value.Name)

		tag := value.Tag
		logger.Debugf("tag: %s", tag)
		if tag.Get("readonly") == "true" {
			logger.Debugf("readonly, go to next one")
			continue
		}

		oldField := reflect.Indirect(reflect.ValueOf(existingEntity).Field(i))
		newField := reflect.Indirect(reflect.ValueOf(newEntity).FieldByName(value.Name))
		logger.Debugf("type: %v", value.Type.Kind())
		if value.Type.Kind() == reflect.Struct {
			logger.Debugf("value: %v", oldField)
			//struct, go into it
			method(oldField.Interface(), newField.Interface())
		} else {
			if oldField != newField && oldField.String() != newField.String() {
				logger.Debugf("Type of old field: %v", oldField.Type())
				logger.Debugf("Type of new field: %v", newField.Type())
				logger.Debugf("Update: %v \nTo %v", oldField, newField)
				oldField.Set(newField) //HERE I get the exception
			} else {
				logger.Debugf("Field values are equal")
			}
		}
	}
}

But when I'm trying to assign a new value with oldField.Set(newField), I get an exception:

reflect: reflect.Value.Set using unaddressable value

I've also tried reflect.ValueOf(existingEntity).Field(i).Set(reflect.ValueOf(newEntity).FieldByName(value.Name)) but got the same exception.

Could you explain to me why it happens and how to fix this?

Pass a pointer to the value you are updating. Here's the function updated to take a pointer for existingEntity:

func method(existingEntity interface{}, newEntity interface{}) {
  entityType := reflect.TypeOf(existingEntity).Elem()
  for i := 0; i < entityType.NumField(); i++ {
	value := entityType.Field(i)
	tag := value.Tag
	if tag.Get("readonly") == "true" {
		continue
	}
	oldField := reflect.ValueOf(existingEntity).Elem().Field(i)
	newField := reflect.ValueOf(newEntity).FieldByName(value.Name)
	if value.Type.Kind() == reflect.Struct {
		method(oldField.Addr().Interface(), newField.Interface())
	} else {
		oldField.Set(newField)
	}
  }
}

Use it like this:

var a Example
b := Example{123, 456, Example2{789}}
method(&a, b)

Run it in the playground

This handles the specific case in the question. The solution is more complicated if a deep clone is required.