Here is the model code for the same:

package models

import (
    "errors"
    "fmt"
    "regexp"

    "gorm.io/gorm"
)

type User struct {
    ID               uint              `gorm:"primaryKey;autoIncrement" json:"id"`
    FirstName        string            `gorm:"column:first_name" json:"first_name"`
    LastName         string            `gorm:"column:last_name" json:"last_name"`
    Email            string            `gorm:"column:email" json:"email"`
    Bio              string            `gorm:"column:bio" json:"bio"`
    Blogs            []Blog            `gorm:"foreignKey:author_id"`
    BlogReactions    []BlogReaction    `gorm:"foreignKey:user_id"`
    CommentReactions []CommentReaction `gorm:"foreignKey:user_id"`
    Comments         []Comment         `gorm:"foreignKey:user_id"`
    Followers        []User            `gorm:"many2many:has_followers"`
}

func (*User) TableName() string {
    return "user"
}

func (u *User) validate() (err error) {

    fmt.Println(u)

    if u.FirstName == "" {
        return errors.New("first name cannot be empty")
    }

    emailMatched, _ := regexp.MatchString("^[\w-\.]+@([\w-]+\.)+[\w-]{2,4}$", u.Email)

    if !emailMatched {
        return errors.New("invalid email format")
    }

    if len(u.Bio) > 200 {
        return errors.New("bio too long")
    }

    return
}

func (u *User) BeforeSave(tx *gorm.DB) (err error) {
    validationErr := u.validate()
    return validationErr
}

I have a validation check before save. When I try to append a new follower:

    dbErr := database.DB.
        Model(&models.User{ID: uint(id)}).
        Association("Followers").
        Append(&models.User{ID: uint(follower_id)})

I get the validation error since the object has no information other than the ID:

first name cannot be empty

If the primary key is present in the object fed into Model() shouldn’t the other fields auto populate? How can I resolve this?