How to Add Choices to database field using GORM

I'm very new to Golang as I mostly code in Python, so came across one problem that I'm not able to solve. I want to add choices to a field in Golang Struct via GORM or any other way if I can achieve that.

My model looks like this

type User struct{
   FirstName string  `gorm:"size:100" json:"first_name"`
   LastName  string  `gorm:"size:100" json:"last_name"`
   Email     *string `gorm:"size:250;index;not null;index:email;unique" json:"email"`
   Role      string  `gorm:"default:User;not null" json:"is_active"` // TODO Add Choice
}

I want to add choices to my Role Field in User Models from ['Admin', 'User', 'Guest']. Is there a way to achieve it using GORM or any other method that will solve my problem? I can make it a Foreign key if there is no direct way to do it. I'm using PostgreSQL to store tables. Thanks in Advance!

Assuming you have a one-to-one relation between roles and users, one approach with the foreign keys would be something like this:

type User struct{
   ID        int64 `json:"id"`
   FirstName string  `gorm:"size:100" json:"first_name"`
   LastName  string  `gorm:"size:100" json:"last_name"`
   Email     *string `gorm:"size:250;index;not null;index:email;unique" json:"email"`
   RoleID    int64  `json:"role_id"` 
   Role      *Role `json:"role"`
}

type Role struct {
   ID   int64 `json:"id"`
   Name string `json:"name"`
}

//load data
var users []User
err := db.Preload("Role").Find(&users).Error

EDIT: solution for many-to-many
relationship

Assuming that you would have a table like users_roles that links the users and roles tables, one solution might look like this:

type User struct{
   ID        int64 `json:"id"`
   FirstName string  `gorm:"size:100" json:"first_name"`
   LastName  string  `gorm:"size:100" json:"last_name"`
   Email     *string `gorm:"size:250;index;not null;index:email;unique" json:"email"`
   Roles      []*Role `gorm:"many2many:users_roles;" json:"roles"`
}

type Role struct {
   ID   int64 `json:"id"`
   Name string `json:"name"`
}

//load data
var users []User
err := db.Preload("Roles").Find(&users).Error

For PostGreSQL First create enum manually in your database

CREATE TYPE user_access AS ENUM (
	'admin',
	'user'
);

Then

type Role string

const (
	Admin  Role = "admin"
	User   Role = "user"
)

func (r *Role) Scan(value interface{}) error {
	*r = Role(value.([]byte))
	return nil
}

func (r Role) Value() (driver.Value, error) {
	return string(r), nil
}

type User struct{
   ID        int64   `json:"id"`
   FirstName string  `gorm:"size:100" json:"first_name"`
   LastName  string  `gorm:"size:100" json:"last_name"`
   Email     *string `gorm:"size:250;index;not null;index:email;unique" json:"email"`
   Role      Role    `sql:"type:user_access"` // PostGreSQL
   Role      Role    `json:"role" sql:"type:ENUM('admin', 'user')"` // MySQL
}

For more info you can check github issue and doc