缺少外键的 GORM "Has Many" 关联？

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按照 gorm 文档 (http://gorm.io/docs/has_many.html#has-many) 中的示例，我尝试创建具有一对多关系的 user 和 creditcard 模型：

package main

import (
    "github.com/jinzhu/gorm"
    _ "github.com/jinzhu/gorm/dialects/sqlite"
    "github.com/sirupsen/logrus"
)

type user struct {
    gorm.model
    creditcards []creditcard
}

type creditcard struct {
    gorm.model
    number string
    userid uint
}

func main() {
    db, err := gorm.open("sqlite3", "examplegorm.db")
    if err != nil {
        logrus.fatalf("open db: %v", err)
    }
    defer db.close()

    db.logmode(true)
    db.automigrate(&user{})
    db.automigrate(&creditcard{})
}

但是，在生成的记录的 sql 语句中，我没有看到类似的内容

create table credit_cards(
    ...
    foreign key(user_id) references users(id)
)

正如我从 sqlite 文档中所期望的那样（https://www.sqlite.org/foreignkeys.html）。我看到的是

> go run gorm_has_many.go

(/users/kurt/documents/scratch/gorm_has_many.go:28) 
[2020-01-06 09:05:58]  [0.82ms]  create table "users" ("id" integer primary key autoincrement,"created_at" datetime,"updated_at" datetime,"deleted_at" datetime )  
[0 rows affected or returned ] 

(/users/kurt/documents/scratch/gorm_has_many.go:28) 
[2020-01-06 09:05:58]  [0.55ms]  create index idx_users_deleted_at on "users"(deleted_at)   
[0 rows affected or returned ] 

(/users/kurt/documents/scratch/gorm_has_many.go:29) 
[2020-01-06 09:05:58]  [0.52ms]  create table "credit_cards" ("id" integer primary key autoincrement,"created_at" datetime,"updated_at" datetime,"deleted_at" datetime,"number" varchar(255),"user_id" integer )  
[0 rows affected or returned ] 

(/users/kurt/documents/scratch/gorm_has_many.go:29) 
[2020-01-06 09:05:58]  [0.50ms]  create index idx_credit_cards_deleted_at on "credit_cards"(deleted_at)   
[0 rows affected or returned ]

相比之下，在 djangoapp 应用程序的示例 django 项目中，如果我有以下 models.py：

from django.db import models


class user(models.model):
    name = models.charfield(max_length=255)
    age = models.positiveintegerfield()


class creditcard(models.model):
    number = models.charfield(max_length=255)
    user = models.foreignkey('djangoapp.user', on_delete=models.cascade)

我检查了与创建 creditcard 模型相对应的 sql 语句，我确实看到了外键引用：

kurt@kurts-macbook-pro-13 ~/d/s/djangoproject> python manage.py sqlmigrate djangoapp 0002_creditcard
(0.000) 
            select name, type from sqlite_master
            where type in ('table', 'view') and not name='sqlite_sequence'
            order by name; args=none
(0.000) select "django_migrations"."id", "django_migrations"."app", "django_migrations"."name", "django_migrations"."applied" from "django_migrations"; args=()
(0.000) pragma foreign_keys = off; args=none
(0.000) pragma foreign_keys; args=none
(0.000) begin; args=none
create table "djangoapp_creditcard" ("id" integer not null primary key autoincrement, "number" varchar(255) not null, "user_id" integer not null references "djangoapp_user" ("id") deferrable initially deferred); (params none)
(0.000) pragma foreign_key_check; args=none
create index "djangoapp_creditcard_user_id_eca64c45" on "djangoapp_creditcard" ("user_id"); (params ())
(0.000) pragma foreign_keys = on; args=none
begin;
--
-- create model creditcard
--
create table "djangoapp_creditcard" ("id" integer not null primary key autoincrement, "number" varchar(255) not null, "user_id" integer not null references "djangoapp_user" ("id") deferrable initially deferred);
create index "djangoapp_creditcard_user_id_eca64c45" on "djangoapp_creditcard" ("user_id");
commit;

为什么没有像 django 应用程序中那样使用 references 子句生成 user_id 列？ （gorm 似乎有一个 addforeignkey() 方法（https://github.com/jinzhu/gorm/blob/79a77d771dee4e4b60e9c543e8663bbc80466670/scope.go#l1229-l1238），但在本例中没有被调用）。

更新

由于 gorm 的 addforeignkey() 方法实际上并未在 gorm 本身内被调用，所以我添加了以下行：

db.model(&creditcard{}).addforeignkey("user_id", "users(id)", "cascade", "cascade")

但是，重新运行脚本似乎显示语法错误：

(/Users/kurt/Documents/Scratch/gorm_has_many.go:30) 
[2020-01-06 09:36:29]  near "CONSTRAINT": syntax error 

(/Users/kurt/Documents/Scratch/gorm_has_many.go:30) 
[2020-01-06 09:36:29]  [0.06ms]  ALTER TABLE "credit_cards" ADD CONSTRAINT credit_cards_user_id_users_id_foreign FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE ON UPDATE CASCADE;  
[0 rows affected or returned ]

解决方案

Gorm 在自动迁移时不添加外键。你必须手动完成。

SQLite不支持在创建表后添加约束。这就是您遇到错误的原因。检查此资源：How do I add a foreign key to an existing SQLite table?

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