处理复杂的关系

处理复杂关系时，涉及多个表之间的关联。在本文中，我们将讨论如何处理现有的表之间的关联，并解决常见的错误，例如空结果。我们将探讨外键设置、多对多关系的设置，以及使用预加载方法从相关表中获取数据。通过遵循本文中的步骤，您将能够正确设置表关联并避免关联查询中常见的错误。

我有 3 个已经存在的表，现在我想要关联，但我所做的一切都返回空 问题类别为空 有接口表字段（app_products_playlist_category）： id 播放列表id 类别_id

我认为问题出在外键上，但我不知道如何为现有表设置它。

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，“创建时间”：“2021-12-28t19：15：38.761224+03：30”，“更新时间”：“0001-01-01t00：00：00z”，“删除时间”：空， "id":9,"已发布":"2","type_playlist1":"","type_playlist2":"","type_playlist3":"2","type_playlist4":"2","价格":0, "price_orginal":1000,"main_img":"playlist/reza/download.jfif","userid":2,"likes_count":"","views_count":"","content":"标题","标题":"分类","类别":null},{"id":0,"创建时间":"2022-01-03t15:13:11.485185+03:30","更新时间":"0001-01-01t00:00:00z","删除时间":null,"id":10,"已发布":"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":"sssssssdasd","title":"电影 2","categories":null}]

标题

正确答案

假设您使用 categories 创建了 playlistmodel，如下所示：

db.create(&playlistmodel{
        published: time.now().string(),
        categories: []categorymodel{
            {name: "test-category"},
        },
    })

要执行返回 playlistmodel 和其中类别的查询，您需要使用 preload 方法，如下所示：

var playlist []*playlistmodel
    db.preload("categories").find(&playlist)

添加：

;joinforeignkey:playlist_id;joinreferences:category_id

至

categories     []categorymodel

如下所示：

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"`
}

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