gorm.DB 可以自动解析外键吗？

Question

I'm trying to use https://github.com/jinzhu/gorm to automatically map foreign keys for me but somehow either I'm doing it wrong or the library cannot do it and I'm in the wrong.

I have the following struct s:

type Currency struct {
  ID           uint64 `gorm:"primary_key"`
  CurrencyCode string `gorm:"size:3"`
}

type Rate struct {
  ID          uint64 `gorm:"primary_key"`
  CurrencyID  uint64
  Currency    Currency `gorm:"ForeignKey:CurrencyID"`
  Price       float64
}

and the following SQL tables ( edited so that currency_code is unique )

CREATE TABLE `rates` (
  `id` serial PRIMARY KEY,
  `currency_id` bigint unsigned NOT NULL,
  `price` decimal(12,2) NOT NULL,
  CONSTRAINT `fk_rate_currency`
    FOREIGN KEY (currency_id) REFERENCES currencies(id)
);

CREATE TABLE `currencies` (
  `id` serial PRIMARY KEY,
  `currency_code` char(3) NOT NULL UNIQUE
);

Now I thought that when I do something like this:

rate := Rate{
  Currency: Currency{
    CurrencyCode: "USD",
  },
  Price: 123,
}
db, _  := gorm.Open("mysql", ...)
db.Create(&rate)

then "USD" would get automatically mapped to a currency_id but instead it inserts new "USD" entry in currencies table for every db.Create(&rate)

Am I doing it wrong or it's the library?

EDIT

I know that I can do it by querying the DB for currency ID

curr := db.Currency{}
db.Where("currency_code = ?", "USD").First(&curr)
// use curr with proper ID

but then:

• I have to make 2 DB calls
• I don't use gorm 's foreign key mapping feature (if there is one)

Answer 1

In their documentation it states:

By default when creating/updating a record, GORM will save its associations, if the association has primary key, GORM will call Update to save it, otherwise it will be created.

So when you want to map to a certain entry you must correlate it with the ID, otherwise it will create a new record.

In your case you could use the currency_code as the PRIMARY KEY for Currency (use fixed number of chars to make it faster, not varchar, like sql:"type:char(3);unique" ).

This would eliminate the need of a search by ID anymore, it would just find "USD" as primary key and use it => no more multiple records per currency. Also if there is a currency that it does not have, it will create it in the Currency table.

Also you can eliminate the gorm:"ForeignKey:CurrencyID" struct tag and let GORM create it's own tables with AutoMigrate , and would create the tables with the appropriate FK:

db := gorm.Open("mysql", ...)
db.Set("gorm:table_options", "ENGINE=InnoDB")
db.Set("gorm:table_options", "collation_connection=utf8_general_ci")
// Migrate the schema
db.AutoMigrate(&models.Currency{})
db.AutoMigrate(&models.Rate{})

Answer 2

In recently released GORM 2.0, foreign keys get added to your database automatically provided your GORM tags are correct. You can say AutoMigrate got smarter.

Just upgrade and use the new imports

go get gorm.io/gorm

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

PS: There are some breaking changes with the new version, kindly check the official relase note for more info https://gorm.io/docs/v2_release_note.html