Structocaster

Allows you to extract only the required fields from the structures and present them in the required format

type User struct {
	ID          int
	Name        string
	Surname     string
	Certificate Passport
}
type Passport struct {
	Serial      string
	CityOfBirth CityOfBirth
}
type CityOfBirth struct {
	Name string
}

type UserToReport struct {
	UID  int
	Name string
	G    string
	From City
}

type City struct {
	Title string `out:"Certificate.CityOfBirth.Name"`
}

Required fields are specified using tags or taken with a matching name from the same nesting level

UserFromDB := User{
	ID:      12,
	Name:    "Aleksei",
	Surname: "Kovrigin",
	Certificate: Passport{
		Serial: "DF374-23479",
		CityOfBirth: CityOfBirth{
			Name: "Kirov",
		},
	},
}

As := City{Title: ""}
UserToReport := UserToReport{
	UID:  13,
	Name: "",
	From: As,
}

structocaster.Cast(&UserFromDB, &UserToReport)

In the issue:

{
        "UID": 13,
        "Name": "Aleksei",
        "G": "",
        "From": {
                "Title": "Kirov"
        }
}