如何用GORM+Postgres存储值数组？迁移多对多模型遇错求助

问题重现 #

参考GORM多对多相关文档编写代码后，运行迁移触发如下错误：

go/src/testpostgres/main.go:25 ERROR: there is no unique constraint matching given keys for referenced table "users" (SQLSTATE 42830)
[4.951ms] [rows:0] CREATE TABLE "user_languages" ("user_id" bigint,"language_id" bigint,PRIMARY KEY ("user_id","language_id"),CONSTRAINT "fk_user_languages_user" FOREIGN KEY ("user_id") REFERENCES "users"("id"),CONSTRAINT "fk_user_languages_language" FOREIGN KEY ("language_id") REFERENCES "languages"("id"))
panic: error while trying to migrate models: ERROR: there is no unique constraint matching given keys for referenced table "users" (SQLSTATE 42830)

用到的代码如下：

main.go #

package main

import (
    "fmt"

    "gorm.io/driver/postgres"
    "gorm.io/gorm"
)

type User struct {
    gorm.Model
    Languages []Language `gorm:"many2many:user_languages;"`
}

type Language struct {
    gorm.Model
    Name string
}

func main() {
    dsn := "host=localhost user=postgres password=asdf dbname=postgres port=5432"
    db, err := gorm.Open(postgres.Open(dsn), &gorm.Config{})
    check("connect to db", err)

    err = db.AutoMigrate(&User{}, &Language{})
    check("migrate models", err)
}

func check(task string, err error) {
    if err != nil {
        panic(fmt.Sprintf("error during %v: %v", task, err.Error()))
    }
}

go.mod #

module testpostgres

go 1.21.1

require (
    gorm.io/driver/postgres v1.5.2
    gorm.io/gorm v1.25.4
)

// ** + indirect requirements ** //

解决方案 #

错误核心是PostgreSQL无法找到users表中id字段的唯一约束（主键约束属于唯一约束的一种），可通过以下方式解决：

1. 重置测试数据库表
   若为测试环境，直接删除已存在的关联表，让GORM重新生成带正确约束的表：

   DROP TABLE IF EXISTS user_languages;
   DROP TABLE IF EXISTS users;
   DROP TABLE IF EXISTS languages;
   

   执行完后重新运行go run main.go即可。

2. 手动添加主键约束
   若无法删除表，直接通过SQL为users表的id字段补全主键约束：

   ALTER TABLE users ADD PRIMARY KEY (id);
   

   完成后再次运行迁移代码即可。

3. 显式声明主键字段
   若AutoMigrate自动生成的约束存在异常，可在结构体中显式声明主键，避免逻辑歧义：

   type User struct {
       ID        uint           `gorm:"primaryKey"`
       CreatedAt time.Time
       UpdatedAt time.Time
       DeletedAt gorm.DeletedAt `gorm:"index"`
       Languages []Language     `gorm:"many2many:user_languages;"`
   }
   

内容的提问来源于stack exchange，提问作者frizzkitten