gorm many to many proplem

I have 3 tables that already exist, now I want to relation , but everything I do comes back empty
proplem category is null
There are interface table fields (app_products_playlist_category):
id
playlist_id
category_id

I think the problem is with foregin key, but I do not know how to set it for existing tables.

type PlayListModel struct {
gorm.Model
Id             int             `gorm:"column:id" json:"id"`
Published      string          `gorm:"column:published" json:"published"`
Type_PlayLIST1 string          `gorm:"column:type_playlist1" json:"type_playlist1"`
Type_PlayLIST2 string          `gorm:"column:type_playlist2" json:"type_playlist2"`
Type_PlayLIST3 string          `gorm:"column:type_playlist3" json:"type_playlist3"`
Type_PlayLIST4 string          `gorm:"column:type_playlist4" json:"type_playlist4"`
Price          int             `gorm:"column:price"  json:"price"`
Price_Orginal  int             `gorm:"column:price_orginal" json:"price_orginal"`
Main_img       string          `gorm:"column:main_img" json:"main_img"`
UserId         int             `gorm:"column:user_id"`
LIkesCount     string          `gorm:"column:likes_count" json:"likes_count"`
ViewsCount     string          `gorm:"column:views_count" json:"views_count"`
Content        string          `gorm:"column:content"  json:"content"`
Title          string          `gorm:"column:title" json:"title"`
Categories     []CategoryModel `gorm:"many2many:app_products_playlist_category;foreignKey:ID;references:ID"`
}
func (playlist *PlayListModel) TableName() string {
return "app_products_playlist"
}
func GetPlayList(playlist *[]PlayListModel, order string) {
Db.Order(order).Find(&playlist)
}
package models
import "gorm.io/gorm"
type CategoryModel struct {
gorm.Model
Id       string          `grom:column:"id" json:"id"`
Name     string          `grom:column:"name" json:"name"`
Img      string          `grom:column:"img" json:"img"`
Slug     string          `grom:column:"slug" json:"slug"`
Row      string          `grom:column:"row" json:"row"`
ParenID  string          `grom:column:"parent_id" json:"parent_id"`
PlayList []PlayListModel `gorm:"many2many:app_products_playlist_category"`
}
func (category *CategoryModel) TableName() string {
return "app_category_category"
}
package models
import "gorm.io/gorm"
type PlayListCategoryModel struct {
PlayListID int `gorm:"primary_key;column:playlist_id" json:"playlist_id"`
CategoryID int `gorm:"primary_key;column:category_id" json:"category_id"`
PlayListModel
CategoryModel
}
func (PlayListCategoryModel) TableName() string {
return "app_products_playlist_category"
}
func (PlayListCategoryModel) BeforeCreate(db *gorm.DB) error {
err := db.SetupJoinTable(&PlayListModel{}, "Categories", &PlayListCategoryModel{})
return err
}

-- auto-generated definition
create table app_products_playlist
(
id             serial
primary key,
published      varchar(10),
title          varchar(255),
type_playlist1 varchar(255),
type_playlist2 varchar(255),
price          integer,
price_orginal  integer,
main_img       varchar(100)             not null,
created_at     timestamp with time zone not null,
user_id        integer
constraint app_products_playlist_user_id_9fb269b5_fk_app_account_user_id
references app_account_user
deferrable initially deferred,
likes_count    integer,
type_playlist3 varchar(50),
views_count    integer,
content        text,
type_playlist4 varchar(50),
deleted_at     timestamp with time zone
);
alter table app_products_playlist
owner to honardari;
create index app_products_playlist_user_id_9fb269b5
on app_products_playlist (user_id);
-- auto-generated definition
create table app_category_category
(
id        serial
primary key,
name      varchar(255),
important varchar(5),
img       varchar(100),
pub_date  date,
slug      varchar(50),
row       integer not null,
lft       integer not null
constraint app_category_category_lft_check
check (lft >= 0),
rght      integer not null
constraint app_category_category_rght_check
check (rght >= 0),
tree_id   integer not null
constraint app_category_category_tree_id_check
check (tree_id >= 0),
level     integer not null
constraint app_category_category_level_check
check (level >= 0),
parent_id integer
constraint app_category_categor_parent_id_0984d0d6_fk_app_categ
references app_category_category
deferrable initially deferred
);
alter table app_category_category
owner to honardari;
create index app_category_category_slug_6027a7e5
on app_category_category (slug);
create index app_category_category_slug_6027a7e5_like
on app_category_category (slug varchar_pattern_ops);
create index app_category_category_tree_id_35de0819
on app_category_category (tree_id);
create index app_category_category_parent_id_0984d0d6
on app_category_category (parent_id);
-- auto-generated definition
create table app_products_playlist_category
(
id          serial
primary key,
playlist_id integer not null
constraint app_products_playlis_playlist_id_f13be1da_fk_app_produ
references app_products_playlist
deferrable initially deferred,
category_id integer not null
constraint app_products_playlis_category_id_d6c7fd45_fk_app_categ
references app_category_category
deferrable initially deferred,
constraint app_products_playlist_ca_playlist_id_category_id_88f46dfa_uniq
unique (playlist_id, category_id)
);
alter table app_products_playlist_category
owner to honardari;
create index app_products_playlist_category_playlist_id_f13be1da
on app_products_playlist_category (playlist_id);
create index app_products_playlist_category_category_id_d6c7fd45
on app_products_playlist_category (category_id);

>[{"ID":0,"CreatedAt":"2021-12-28T19:15:38.761224+03:30","UpdatedAt":"0001-01-01T00:00:00Z","DeletedAt":null,"id":9,"published":"2","type_playlist1":"","type_playlist2":"","type_playlist3":"2","type_playlist4":"2","price":0,"price_orginal":1000,"main_img":"playlist/reza/download.jfif","UserId":2,"likes_count":"","views_count":"","content":"تست تست تست تست تستست تستست تستست","title":"آموزش کیف چرم","Categories":null},{"ID":0,"CreatedAt":"2022-01-03T15:13:11.485185+03:30","UpdatedAt":"0001-01-01T00:00:00Z","DeletedAt":null,"id":10,"published":"2","type_playlist1":"","type_playlist2":"","type_playlist3":"2","type_playlist4":"2","price":0,"price_orginal":2500,"main_img":"playlist/09301292813/QYfe8BCL.1.jpg","UserId":1,"likes_count":"","views_count":"","content":"سشدیندشسیسشsss ssdasd","title":"آموزش کیف چرم 2","Categories":null}]

Heading

Supposing that you've created a playlistModel with categories, like this:

db.Create(&PlayListModel{
Published: time.Now().String(),
Categories: []CategoryModel{
{Name: "test-category"},
},
})

For do a query that returns playlistModel and categories inside, you need use Preload method, in this way:

var playlist []*PlayListModel
db.Preload("Categories").Find(&playlist)

Add:

;joinForeignKey:playlist_id;joinReferences:category_id

to

Categories     []CategoryModel 

As done in the following:

type PlayListModel struct {
gorm.Model
Id             int             `gorm:"column:id" json:"id"`
Published      string          `gorm:"column:published" json:"published"`
Type_PlayLIST1 string          `gorm:"column:type_playlist1" json:"type_playlist1"`
Type_PlayLIST2 string          `gorm:"column:type_playlist2" json:"type_playlist2"`
Type_PlayLIST3 string          `gorm:"column:type_playlist3" json:"type_playlist3"`
Type_PlayLIST4 string          `gorm:"column:type_playlist4" json:"type_playlist4"`
Price          int             `gorm:"column:price"  json:"price"`
Price_Orginal  int             `gorm:"column:price_orginal" json:"price_orginal"`
Main_img       string          `gorm:"column:main_img" json:"main_img"`
UserId         int             `gorm:"column:user_id"`
LIkesCount     string          `gorm:"column:likes_count" json:"likes_count"`
ViewsCount     string          `gorm:"column:views_count" json:"views_count"`
Content        string          `gorm:"column:content"  json:"content"`
Title          string          `gorm:"column:title" json:"title"`
Categories     []CategoryModel `gorm:"many2many:app_products_playlist_category;joinForeignKey:playlist_id;joinReferences:category_id"`
}