Support the AUTO_RANDOM Field

[Icemap]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

For this issue: go-gorm/gorm#5977
Support the AUTO_RANDOM.

User Case Description

If you already deploy a TiDB at localhost:4000. You can use this code to test.

package main

import (
	"fmt"
	"gorm.io/driver/mysql"
	"gorm.io/gorm"
)

type Product struct {
	ID    uint `gorm:"primaryKey;default:auto_random()"`
	Code  string
	Price uint
}

func main() {
	db, err := gorm.Open(mysql.Open("root:@tcp(127.0.0.1:4000)/test"), &gorm.Config{})
	if err != nil {
		panic("failed to connect database")
	}

	db.AutoMigrate(&Product{})

	insertProduct := &Product{Code: "D42", Price: 100}

	db.Create(insertProduct)
	fmt.Printf("insert ID: %d, Code: %s, Prict: %d\n",
		insertProduct.ID, insertProduct.Code, insertProduct.Price)

	readProduct := &Product{}
	db.First(&readProduct, "code = ?", "D42") // find product with code D42

	fmt.Printf("read ID: %d, Code: %s, Prict: %d\n",
		readProduct.ID, readProduct.Code, readProduct.Price)
}

[Icemap]

Change these two PR's implementation methods:

• Support the AUTO_RANDOM Field #102
• Support the AUTO_RANDOM Field gorm#5981

For avoiding interface changes.

[Icemap]

PTAL @jinzhu

[jinzhu]

Why do we need to set HasDefaultValue to false, it should be true?

[Icemap]

We used this tag default:auto_random() to implement the auto_random feature. But it is actually not a default value. It's a field like auto_increment.

[jinzhu]

for auto_increment fields we also marked them as HasDefaultValue, e.g:

https://github.com/go-gorm/gorm/blob/d834dd60b715422dc2a900fb2744f9c278a9830f/schema/field.go#L113

[Icemap]

Ah, got it. I made a mistake.

[Icemap]

Fixed. PTAL again.