Your Question

Say for example I have these structs:

type Person struct {
  ID        int
  Name      string
  Addresses []Address `gorm:"many2many:person_addresses;"`
}

type Address struct {
  ID   uint
  Name string
}

type PersonAddress struct {
  PersonID  int
  AddressID int
  FieldOne string
  FieldTwo string
}

[ ... ]

_ = DB.SetupJoinTable(&Person{}, "Addresses", &PersonAddress{})

My question is:

a) how do I select these custom field out of the join table to get them as JSON when selecting out Person with Preload of Addresses b) how do I save new values to these, the documentation says you can have custom fields but doesn't explain how to actually do this

The document you expected this should be explained

https://gorm.io/docs/many_to_many.html#Customize-JoinTable

Expected answer

Documentation describing how this could be done?

Comment From: jinzhu

a) how do I select these custom field out of the join table to get them as JSON when selecting out Person with Preload of Addresses

Preload won't query join table's content, use the join table as has-many to load its value.

b) how do I save new values to these, the documentation says you can have custom fields but doesn't explain how to actually do this

When using many2many, GORM will only create the record with user, address's foreign keys, you could add other data in PersonAddress's before save hooks.

Comment From: BogdanJava

@jinzhu Regarding the point b): when I try to insert into projects it says that the join table has no column roles, it only creates the key fields although I set a value for this column

package main

import (
    "gorm.io/driver/sqlite"
    "gorm.io/gorm"
)

type User struct {
    ID uint
    Name string
}

type ProjectUser struct {
    UserID uint
    ProjectID uint
    Roles string 
}

func (user *ProjectUser) BeforeSave(db *gorm.DB) error {
    user.Roles = "default_val"
    return nil
}

type Project struct {
    ID uint
    Name string
    Users []User `gorm:"many2many:project_users;"`
}

func main() {
    db, err := gorm.Open(sqlite.Open("test4.db"), &gorm.Config{})
    if err != nil {
        panic("failed to connect database")
    }

    db.AutoMigrate(&User{}, &Project{})

    if err := db.SetupJoinTable(&Project{}, "Users", &ProjectUser{}); err != nil {
        println(err.Error())
        panic("Failed to setup join table")
    }

    bogdan := User { Name: "Bogdan" }

    db.Create(&bogdan)

    db.Create(&Project{Name: "Test1", Users: []User {{ID: bogdan.ID}}})
}

go run main.go

2021/08/19 16:32:18 /Users/bahdan.shyshkin/work/gorm_test/main.go:47 table project_users has no column named roles
[0.020ms] [rows:0] INSERT INTO `project_users` (`user_id`,`project_id`,`roles`) VALUES (1,1,"default_val") ON CONFLICT DO NOTHING

2021/08/19 16:32:18 /Users/bahdan.shyshkin/work/gorm_test/main.go:47 table project_users has no column named roles
[0.634ms] [rows:1] INSERT INTO `projects` (`name`) VALUES ("Test1")

Comment From: shreeyashnaik

@BogdanJava same is happening with me. @jinzhu what can be the issue?

Comment From: shreeyashnaik

@BogdanJava I've figured it out. Add the SetupJoinTable method just before Automigrate.

Comment From: ranusingh1993

@BogdanJava I've figured it out. Add the SetupJoinTable method just before Automigrate.

Yes it worked for me, nice solution. Thanks for the help.

Comment From: paulinevos

@BogdanJava @shreeyashnaik Something I still don't understand... In @BogdanJava's example above, how does one populate the roles field on the join table with anything other than default_val, as you can't pass anything to the BeforeCreate hook?

Comment From: aicomyllevillestudent

@apuckey Do you still remember how you solved a)? Or @jinzhu could you give an example of the Join you would use for this situation?

Comment From: birukbelay

yea same here, can you provide example, specially on the official documentation, i was even surprised in the first place not finding it

Comment From: loeffel-io

@BogdanJava @shreeyashnaik Something I still don't understand... In @BogdanJava's example above, how does one populate the roles field on the join table with anything other than default_val, as you can't pass anything to the BeforeCreate hook?

this

Comment From: kein-1

yea same here, can you provide example, specially on the official documentation, i was even surprised in the first place not finding it

Were you able to solve it? I'm confused; same question as you and how to query (a)