# GORM Notes
## Start
* ### Install

go get -u gorm.io/gorm
go get -u gorm.io/driver/sqlite

* ### Models

type User struct {
  ID           uint           // Standard field for the primary key
  Name         string         // A regular string field
  Email        *string        // A pointer to a string, allowing for null values
  Age          uint8          // An unsigned 8-bit integer
  Birthday     *time.Time     // A pointer to time.Time, can be null
  MemberNumber sql.NullString // Uses sql.NullString to handle nullable strings
  ActivatedAt  sql.NullTime   // Uses sql.NullTime for nullable time fields
  CreatedAt    time.Time      // Automatically managed by GORM for creation time
  UpdatedAt    time.Time      // Automatically managed by GORM for update time
}

* *string：pointers to types indicate nullable fields.
* CreatedAt and UpdatedAt are special fields that GORM automatically populates with the current time when a record is created or updated.
#### gorm.model
* GORM provides a predefined struct named gorm.Model.
* Embed gorm.Model directly in your structs to include these fields automatically.

// gorm.Model definition
type Model struct {
  ID        uint           `gorm:"primaryKey"`
  CreatedAt time.Time
  UpdatedAt time.Time
  DeletedAt gorm.DeletedAt `gorm:"index"`
}

#### Embedded Struct

type User struct {
  gorm.Model
  Name string
}
// equals
type User struct {
  ID        uint           `gorm:"primaryKey"`
  CreatedAt time.Time
  UpdatedAt time.Time
  DeletedAt gorm.DeletedAt `gorm:"index"`
  Name string
}

* ### Database - MySQL

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

func main() {
  // refer https://github.com/go-sql-driver/mysql#dsn-data-source-name for details
  dsn := "user:pass@tcp(127.0.0.1:3306)/dbname?charset=utf8mb4&parseTime=True&loc=Local"
  db, err := gorm.Open(mysql.Open(dsn), &gorm.Config{})
}

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

func main() {
  // refer https://github.com/go-sql-driver/mysql#dsn-data-source-name for details
  dsn := "user:pass@tcp(127.0.0.1:3306)/dbname?charset=utf8mb4&parseTime=True&loc=Local"
  db, err := gorm.Open(mysql.Open(dsn), &gorm.Config{})
}

## CRUD Interface
### Create
* Batch Insert

var users = []User{{Name: "jinzhu_1"}, ...., {Name: "jinzhu_10000"}}

// batch size 100
db.CreateInBatches(users, 100)

* Create Hooks
> GORM allows user defined hooks to be implemented for BeforeSave, BeforeCreate, AfterSave, AfterCreate.

func (u *User) BeforeCreate(tx *gorm.DB) (err error) {
  u.UUID = uuid.New()

  if u.Role == "admin" {
    return errors.New("invalid role")
  }
  return
}

* Upsert/On Conflict

import "gorm.io/gorm/clause"

// Update columns to default value on `id` conflict
db.Clauses(clause.OnConflict{
  Columns:   []clause.Column{{Name: "id"}},
  DoUpdates: clause.Assignments(map[string]interface{}{"role": "user"}),
}).Create(&users)
// MERGE INTO "users" USING *** WHEN NOT MATCHED THEN INSERT *** WHEN MATCHED THEN UPDATE SET ***; SQL Server
// INSERT INTO `users` *** ON DUPLICATE KEY UPDATE ***; MySQL

// Update all columns to new value on conflict except primary keys and those columns having default values from sql func
db.Clauses(clause.OnConflict{
  UpdateAll: true,
}).Create(&users)
// INSERT INTO "users" *** ON CONFLICT ("id") DO UPDATE SET "name"="excluded"."name", "age"="excluded"."age", ...;
// INSERT INTO `users` *** ON DUPLICATE KEY UPDATE `name`=VALUES(name),`age`=VALUES(age), ...;

### Raw SQL & SQL Builder
* #### Clauses
>GORM uses SQL builder generates SQL internally, for each operation, GORM creates a gorm.Statement object, all GORM APIs add/change Clause for the Statement, at last, GORM generated SQL based on those clauses.

var limit = 1
clause.Select{Columns: []clause.Column{{Name: "*"}}}
clause.From{Tables: []clause.Table{{Name: clause.CurrentTable}}}
clause.Limit{Limit: &limit}
clause.OrderBy{Columns: []clause.OrderByColumn{
  {
    Column: clause.Column{
      Table: clause.CurrentTable,
      Name:  clause.PrimaryKey,
    },
  },
}}

Statement.Build("SELECT", "FROM", "WHERE", "GROUP BY", "ORDER BY", "LIMIT", "FOR")
//SELECT * FROM `users` ORDER BY `users`.`id` LIMIT 1