Remove Element From Struct But Only For This One Function - go

Question

So I have an Struct that holds data that has a AddedByUser which links to my User Struct.
What I want to be able to do it remove the UserLevel from the AddedByUser
Now I want to be able to do it from this function only, so using the json:"-" is not an option. That would remove it from all json output. I only want to remove it form this one function.
I should also say that these are Gorm models and when I have been trying to remove the 10 option (UserLevels) it only removes the outer data set not the UserLevel from all of the data.
{
"ID": 1,
"CreatedAt": "2019-01-08T16:33:09.514711Z",
"UpdatedAt": "2019-01-08T16:33:09.514711Z",
"DeletedAt": null,
"UUID": "00000000-0000-0000-0000-000000000000",
"Title": "title000",
"Information": "info999",
"EventDate": "2006-01-02T15:04:05Z",
"AddedByUser": {
"ID": 2,
"CreatedAt": "2019-01-08T15:27:52.435397Z",
"UpdatedAt": "2019-01-08T15:27:52.435397Z",
"DeletedAt": null,
"UUID": "b019df80-a7e4-4397-814a-795e7e84b4ca",
"Firstname": "Me",
"Surname": "admin",
"Password": "....",
"Email": "admin#email.co.uk",
"UserLevel": {
"ID": 0,
"CreatedAt": "0001-01-01T00:00:00Z",
"UpdatedAt": "0001-01-01T00:00:00Z",
"DeletedAt": null,
"LevelTitle": "",
"UserLevel": null
},
So this is what I have tried,
data := []models.MyData{}
data = append(data[0:2])
I have about 14 results, with out the append it loads all the results but with this is only loads two results. The idea was to remove either UpdateAt or Title. As I am not sure if the gorm model information is all 0 or if the slice sees them as 0,1,2,3,4 etc.
I have also tried to range over the slice of models, while I can access each of the sections, I can not seem to find a simple method to remove data by name from a struct? Maps seem to have that but not structs which I am not sure why?
Thanks.
UPDATE
This is the model I am using:
//Model
type MyData struct {
gorm.Model
UUID uuid.UUID
Title string
Information string
EventDate time.Time
AddedByUser Users `gorm:"ForeignKey:added_by_user_fk"`
AddedByUserFK uint
}
//Users Model
type Users struct {
gorm.Model
UUID uuid.UUID
Firstname string
Surname string
Password string
Email string
UserLevel UserLevels `gorm:"ForeignKey:user_level_fk" json:",omitempty"`
UserLevelFK uint
}

Answer 1

As mentioned in the comments, you cannot remove fields from a struct value, because that would yield a value of a different type.
However, you can set fields to their zero value. Combined with the omitempty JSON tag, you can exclude fields from the JSON encoding. To make this work properly, you have to change the UserLevel field to a pointer type (otherwise you end up with empty objects in the JSON document).
Types shortened for brevity:
package main
import (
"encoding/json"
"fmt"
)
type MyData struct {
Title string
AddedByUser Users
}
type Users struct {
ID int
UserLevel *UserLevels `json:",omitempty"` // pointer type with omitempty
}
type UserLevels struct {
LevelTitle string
}
func main() {
var x MyData
x.Title = "foo"
x.AddedByUser.ID = 2
x.AddedByUser.UserLevel = &UserLevels{}
f(x)
b, _ := json.MarshalIndent(x, "", " ")
fmt.Println("main:\n" + string(b))
}
func f(x MyData) {
// "unset" UserLevel. Since we are receiving a copy of MyData, this is
// invisible to the caller.
x.AddedByUser.UserLevel = nil
b, _ := json.MarshalIndent(x, "", " ")
fmt.Println("f:\n" + string(b))
}
// Output:
// f:
// {
// "Title": "foo",
// "AddedByUser": {
// "ID": 2
// }
// }
// main:
// {
// "Title": "foo",
// "AddedByUser": {
// "ID": 2,
// "UserLevel": {
// "LevelTitle": ""
// }
// }
// }
Try it on the playground: https://play.golang.org/p/trUgnYamVOA
Alternatively, you can define new types that exclude the AddedByUser field. However, since this field isn't at the top level, this is a lot of work, and it's easy to forget to update those types when new fields are added to the original types.
If the field were at the top level, the compiler would do most of the work for you, because types that only differ in their field tags can be directly converted to one another:
type MyData struct {
ID int
Title string
}
func main() {
var x MyData
x.ID = 1
x.Title = "foo"
f(x)
}
func f(x MyData) {
type data struct { // same as MyData, except the field tags
ID int
Title string `json:"-"`
}
b, _ := json.MarshalIndent(data(x), "", " ")
fmt.Println("main:\n" + string(b))
}