GORM 基础 -- CRUD 接口（二）

1、Update

1.1 保存所有字段

Save将在执行update SQL时保存所有字段

db.First(&user)

user.Name = "jinzhu 2"
user.Age = 100
db.Save(&user)
// UPDATE users SET name='jinzhu 2', age=100, birthday='2016-01-01', updated_at = '2013-11-17 21:34:10' WHERE id=111;

Save是一个组合函数。如果保存值不包含主键，它将执行Create，否则将执行Update(包含所有字段)。

db.Save(&User{Name: "jinzhu", Age: 100})
// INSERT INTO `users` (`name`,`age`,`birthday`,`update_at`) VALUES ("jinzhu",100,"0000-00-00 00:00:00","0000-00-00 00:00:00")

db.Save(&User{ID: 1, Name: "jinzhu", Age: 100})
// UPDATE `users` SET `name`="jinzhu",`age`=100,`birthday`="0000-00-00 00:00:00",`update_at`="0000-00-00 00:00:00" WHERE `id` = 1

不要 和Model一起使用Save ，这是一个未定义的行为。

1.2 更新单列

当使用Update更新单个列时，它需要有任何条件，否则将引发错误ErrMissingWhereClause，请检查块全局更新以了解详细信息。
当使用Model方法并且它的值有一个主值时，主键将被用于构建条件，例如:

// Update with conditions
db.Model(&User{}).Where("active = ?", true).Update("name", "hello")
// UPDATE users SET name='hello', updated_at='2013-11-17 21:34:10' WHERE active=true;

// User's ID is `111`:
db.Model(&user).Update("name", "hello")
// UPDATE users SET name='hello', updated_at='2013-11-17 21:34:10' WHERE id=111;

// Update with conditions and model value
db.Model(&user).Where("active = ?", true).Update("name", "hello")
// UPDATE users SET name='hello', updated_at='2013-11-17 21:34:10' WHERE id=111 AND active=true;

1.3 更新多列

Updates支持使用struct或map[string]interface{}进行更新，当使用struct进行更新时，默认只更新非零字段

// Update attributes with `struct`, will only update non-zero fields
db.Model(&user).Updates(User{Name: "hello", Age: 18, Active: false})
// UPDATE users SET name='hello', age=18, updated_at = '2013-11-17 21:34:10' WHERE id = 111;

// Update attributes with `map`
db.Model(&user).Updates(map[string]interface{}{"name": "hello", "age": 18, "active": false})
// UPDATE users SET name='hello', age=18, active=false, updated_at='2013-11-17 21:34:10' WHERE id=111;

当使用struc进行更新时，GORM将只更新非零字段。您可能希望使用map更新属性或使用Select指定要更新的字段

1.4 更新所选字段

如果要更新所选字段或在更新时忽略某些字段，可以使用Select, Omit

// Select with Map
// User's ID is `111`:
db.Model(&user).Select("name").Updates(map[string]interface{}{"name": "hello", "age": 18, "active": false})
// UPDATE users SET name='hello' WHERE id=111;

db.Model(&user).Omit("name").Updates(map[string]interface{}{"name": "hello", "age": 18, "active": false})
// UPDATE users SET age=18, active=false, updated_at='2013-11-17 21:34:10' WHERE id=111;

// Select with Struct (select zero value fields)
db.Model(&user).Select("Name", "Age").Updates(User{Name: "new_name", Age: 0})
// UPDATE users SET name='new_name', age=0 WHERE id=111;

// Select all fields (select all fields include zero value fields)
db.Model(&user).Select("*").Updates(User{Name: "jinzhu", Role: "admin", Age: 0})

// Select all fields but omit Role (select all fields include zero value fields)
db.Model(&user).Select("*").Omit("Role").Updates(User{Name: "jinzhu", Role: "admin", Age: 0})

1.5 更新的钩子

GORM允许钩子BeforeSave, BeforeUpdate, AfterSave, AfterUpdate。这些方法将在更新记录时被调用，详细信息请参阅Hooks

func (u *User) BeforeUpdate(tx *gorm.DB) (err error) {
  if u.Role == "admin" {
    return errors.New("admin user not allowed to update")
  }
  return
}

1.6 批量更新

如果我们没有使用Model指定具有主键值的记录，GORM将执行批量更新

// Update with struct
db.Model(User{}).Where("role = ?", "admin").Updates(User{Name: "hello", Age: 18})
// UPDATE users SET name='hello', age=18 WHERE role = 'admin';

// Update with map
db.Table("users").Where("id IN ?", []int{10, 11}).Updates(map[string]interface{}{"name": "hello", "age": 18})
// UPDATE users SET name='hello', age=18 WHERE id IN (10, 11);

块全局更新

如果在没有任何条件的情况下执行批量更新，GORM将不会运行它，默认情况下将返回ErrMissingWhereClause错误

你必须使用一些条件或使用原始SQL或启用AllowGlobalUpdate模式，例如:

db.Model(&User{}).Update("name", "jinzhu").Error // gorm.ErrMissingWhereClause

db.Model(&User{}).Where("1 = 1").Update("name", "jinzhu")
// UPDATE users SET `name` = "jinzhu" WHERE 1=1

db.Exec("UPDATE users SET name = ?", "jinzhu")
// UPDATE users SET name = "jinzhu"

db.Session(&gorm.Session{AllowGlobalUpdate: true}).Model(&User{}).Update("name", "jinzhu")
// UPDATE users SET `name` = "jinzhu"

更新的纪录计数

获取受更新影响的行数

// Get updated records count with `RowsAffected`
result := db.Model(User{}).Where("role = ?", "admin").Updates(User{Name: "hello", Age: 18})
// UPDATE users SET name='hello', age=18 WHERE role = 'admin';

result.RowsAffected // returns updated records count
result.Error        // returns updating error

1.7 高级用法

1.7.1 使用SQL表达式更新

GORM允许用SQL表达式更新列，例如:

// product's ID is `3`
db.Model(&product).Update("price", gorm.Expr("price * ? + ?", 2, 100))
// UPDATE "products" SET "price" = price * 2 + 100, "updated_at" = '2013-11-17 21:34:10' WHERE "id" = 3;

db.Model(&product).Updates(map[string]interface{}{"price": gorm.Expr("price * ? + ?", 2, 100)})
// UPDATE "products" SET "price" = price * 2 + 100, "updated_at" = '2013-11-17 21:34:10' WHERE "id" = 3;

db.Model(&product).UpdateColumn("quantity", gorm.Expr("quantity - ?", 1))
// UPDATE "products" SET "quantity" = quantity - 1 WHERE "id" = 3;

db.Model(&product).Where("quantity > 1").UpdateColumn("quantity", gorm.Expr("quantity - ?", 1))
// UPDATE "products" SET "quantity" = quantity - 1 WHERE "id" = 3 AND quantity > 1;

GORM还允许使用自定义数据类型的SQL Expression/Context Valuer进行更新

// Create from customized data type
type Location struct {
  X, Y int
}

func (loc Location) GormValue(ctx context.Context, db *gorm.DB) clause.Expr {
  return clause.Expr{
    SQL:  "ST_PointFromText(?)",
    Vars: []interface{}{fmt.Sprintf("POINT(%d %d)", loc.X, loc.Y)},
  }
}

db.Model(&User{ID: 1}).Updates(User{
  Name:  "jinzhu",
  Location: Location{X: 100, Y: 100},
})
// UPDATE `user_with_points` SET `name`="jinzhu",`location`=ST_PointFromText("POINT(100 100)") WHERE `id` = 1

1.7.2 从SubQuery更新

Update a table by using SubQuery

db.Model(&user).Update("company_name", db.Model(&Company{}).Select("name").Where("companies.id = users.company_id"))
// UPDATE "users" SET "company_name" = (SELECT name FROM companies WHERE companies.id = users.company_id);

db.Table("users as u").Where("name = ?", "jinzhu").Update("company_name", db.Table("companies as c").Select("name").Where("c.id = u.company_id"))

db.Table("users as u").Where("name = ?", "jinzhu").Updates(map[string]interface{}{"company_name": db.Table("companies as c").Select("name").Where("c.id = u.company_id")})

1.7.3 Without Hooks/Time Tracking

如果你想跳过Hooks方法，并且在更新时不跟踪更新时间，你可以使用UpdateColumn, UpdateColumns，它的工作方式类似update, Updates

// Update single column
db.Model(&user).UpdateColumn("name", "hello")
// UPDATE users SET name='hello' WHERE id = 111;

// Update multiple columns
db.Model(&user).UpdateColumns(User{Name: "hello", Age: 18})
// UPDATE users SET name='hello', age=18 WHERE id = 111;

// Update selected columns
db.Model(&user).Select("name", "age").UpdateColumns(User{Name: "hello", Age: 0})
// UPDATE users SET name='hello', age=0 WHERE id = 111;

1.7.4 从修改的行返回数据

返回更改后的数据只适用于支持返回的数据库，例如:

// return all columns
var users []User
DB.Model(&users).Clauses(clause.Returning{}).Where("role = ?", "admin").Update("salary", gorm.Expr("salary * ?", 2))
// UPDATE `users` SET `salary`=salary * 2,`updated_at`="2021-10-28 17:37:23.19" WHERE role = "admin" RETURNING *
// users => []User{{ID: 1, Name: "jinzhu", Role: "admin", Salary: 100}, {ID: 2, Name: "jinzhu.2", Role: "admin", Salary: 1000}}

// return specified columns
DB.Model(&users).Clauses(clause.Returning{Columns: []clause.Column{{Name: "name"}, {Name: "salary"}}}).Where("role = ?", "admin").Update("salary", gorm.Expr("salary * ?", 2))
// UPDATE `users` SET `salary`=salary * 2,`updated_at`="2021-10-28 17:37:23.19" WHERE role = "admin" RETURNING `name`, `salary`
// users => []User{{ID: 0, Name: "jinzhu", Role: "", Salary: 100}, {ID: 0, Name: "jinzhu.2", Role: "", Salary: 1000}}

1.7.5 检查字段是否已更改?

GORM提供了在Before Update Hooks中使用的Changed方法，它将返回字段是否已更改。

Changed方法只适用于Update、Updates方法，它只检查Update / Updates中的更新值是否等于模型值。如果它被更改且未被省略，则返回true

func (u *User) BeforeUpdate(tx *gorm.DB) (err error) {
  // if Role changed
  if tx.Statement.Changed("Role") {
    return errors.New("role not allowed to change")
  }

  if tx.Statement.Changed("Name", "Admin") { // if Name or Role changed
    tx.Statement.SetColumn("Age", 18)
  }

  // if any fields changed
  if tx.Statement.Changed() {
    tx.Statement.SetColumn("RefreshedAt", time.Now())
  }
  return nil
}

db.Model(&User{ID: 1, Name: "jinzhu"}).Updates(map[string]interface{"name": "jinzhu2"})
// Changed("Name") => true
db.Model(&User{ID: 1, Name: "jinzhu"}).Updates(map[string]interface{"name": "jinzhu"})
// Changed("Name") => false, `Name` not changed
db.Model(&User{ID: 1, Name: "jinzhu"}).Select("Admin").Updates(map[string]interface{
  "name": "jinzhu2", "admin": false,
})
// Changed("Name") => false, `Name` not selected to update

db.Model(&User{ID: 1, Name: "jinzhu"}).Updates(User{Name: "jinzhu2"})
// Changed("Name") => true
db.Model(&User{ID: 1, Name: "jinzhu"}).Updates(User{Name: "jinzhu"})
// Changed("Name") => false, `Name` not changed
db.Model(&User{ID: 1, Name: "jinzhu"}).Select("Admin").Updates(User{Name: "jinzhu2"})
// Changed("Name") => false, `Name` not selected to update

1.7.6 更改更新值

要更改Before Hooks中的更新值，你应该使用SetColumn，除非它是Save的完整更新，例如:

func (user *User) BeforeSave(tx *gorm.DB) (err error) {
  if pw, err := bcrypt.GenerateFromPassword(user.Password, 0); err == nil {
    tx.Statement.SetColumn("EncryptedPassword", pw)
  }

  if tx.Statement.Changed("Code") {
    user.Age += 20
    tx.Statement.SetColumn("Age", user.Age)
  }
}

db.Model(&user).Update("Name", "jinzhu")

2、Delete

2.1 删除一条记录

当删除一条记录时，被删除的值需要有主键，否则将触发批量删除
如果没有软删除标记，则直接删除记录

// Email's ID is `10`
db.Delete(&email)
// DELETE from emails where id = 10;

// Delete with additional conditions
db.Where("name = ?", "jinzhu").Delete(&email)
// DELETE from emails where id = 10 AND name = "jinzhu";

2.2 用主键删除

GORM允许使用带有内联条件的主键删除对象，它适用于数字，详细信息请参阅查询的内联条件

db.Delete(&User{}, 10)
// DELETE FROM users WHERE id = 10;

db.Delete(&User{}, "10")
// DELETE FROM users WHERE id = 10;

db.Delete(&users, []int{1,2,3})
// DELETE FROM users WHERE id IN (1,2,3);

2.3 删除钩子

GORM允许钩子BeforeDelete, AfterDelete，这些方法将在删除记录时被调用，详细信息请参阅钩子

func (u *User) BeforeDelete(tx *gorm.DB) (err error) {
  if u.Role == "admin" {
    return errors.New("admin user not allowed to delete")
  }
  return
}

2.4 批量删除

指定的值没有主值，GORM将执行批量删除，它将删除所有匹配的记录

db.Where("email LIKE ?", "%jinzhu%").Delete(&Email{})
// DELETE from emails where email LIKE "%jinzhu%";

db.Delete(&Email{}, "email LIKE ?", "%jinzhu%")
// DELETE from emails where email LIKE "%jinzhu%";

为了有效地删除大量记录，将一个带主键的片传递给Delete 方法。

var users = []User{{ID: 1}, {ID: 2}, {ID: 3}}
db.Delete(&users)
// DELETE FROM users WHERE id IN (1,2,3);

db.Delete(&users, "name LIKE ?", "%jinzhu%")
// DELETE FROM users WHERE name LIKE "%jinzhu%" AND id IN (1,2,3); 

块全局删除

如果你在没有任何条件的情况下执行批量删除，GORM将不会运行它，并返回ErrMissingWhereClause错误

你必须使用一些条件或使用原始SQL或启用AllowGlobalUpdate模式，例如:

db.Delete(&User{}).Error // gorm.ErrMissingWhereClause

db.Delete(&[]User{{Name: "jinzhu1"}, {Name: "jinzhu2"}}).Error // gorm.ErrMissingWhereClause

db.Where("1 = 1").Delete(&User{})
// DELETE FROM `users` WHERE 1=1

db.Exec("DELETE FROM users")
// DELETE FROM users

db.Session(&gorm.Session{AllowGlobalUpdate: true}).Delete(&User{})
// DELETE FROM users

从已删除的行返回数据

返回已删除的数据，仅适用于数据库支持返回，例如:

// return all columns
var users []User
DB.Clauses(clause.Returning{}).Where("role = ?", "admin").Delete(&users)
// DELETE FROM `users` WHERE role = "admin" RETURNING *
// users => []User{{ID: 1, Name: "jinzhu", Role: "admin", Salary: 100}, {ID: 2, Name: "jinzhu.2", Role: "admin", Salary: 1000}}

// return specified columns
DB.Clauses(clause.Returning{Columns: []clause.Column{{Name: "name"}, {Name: "salary"}}}).Where("role = ?", "admin").Delete(&users)
// DELETE FROM `users` WHERE role = "admin" RETURNING `name`, `salary`
// users => []User{{ID: 0, Name: "jinzhu", Role: "", Salary: 100}, {ID: 0, Name: "jinzhu.2", Role: "", Salary: 1000}}

2.5 软删除

如果你的模型包括一个gorm.DeletedAt字段(包含在gorm.Model中)，它将自动获得软删除能力!

当调用Delete时，记录不会从数据库中删除，但是GORM会将DeletedAt的值设置为当前时间，并且数据不能再用普通的查询方法找到。

// user's ID is `111`
db.Delete(&user)
// UPDATE users SET deleted_at="2013-10-29 10:23" WHERE id = 111;

// Batch Delete
db.Where("age = ?", 20).Delete(&User{})
// UPDATE users SET deleted_at="2013-10-29 10:23" WHERE age = 20;

// Soft deleted records will be ignored when querying
db.Where("age = 20").Find(&user)
// SELECT * FROM users WHERE age = 20 AND deleted_at IS NULL;

如果你不想包括gorm.Model，您可以启用软删除功能，如:

type User struct {
  ID      int
  Deleted gorm.DeletedAt
  Name    string
}

查找软删除记录

你可以用Unscoped找到软删除的记录

db.Unscoped().Where("age = 20").Find(&users)
// SELECT * FROM users WHERE age = 20;

永久删除

您可以使用无作用域永久删除匹配的记录

db.Unscoped().Delete(&order)
// DELETE FROM orders WHERE id=10;

删除标记

默认为gorm.Model使用*time.Time作为DeletedAt字段的值，并且它通过插件gorm.io/plugin/soft_delete提供了其他数据格式支持

当为DeletedAt字段创建唯一的复合索引时，您必须使用其他数据格式，例如插件gorm.io/plugin/soft_delete帮助下的unix second/flag：

import "gorm.io/plugin/soft_delete"

type User struct {
  ID        uint
  Name      string                `gorm:"uniqueIndex:udx_name"`
  DeletedAt soft_delete.DeletedAt `gorm:"uniqueIndex:udx_name"`
}

Unix Second

使用unix Second作为删除标志

import "gorm.io/plugin/soft_delete"

type User struct {
  ID        uint
  Name      string
  DeletedAt soft_delete.DeletedAt
}

// Query
SELECT * FROM users WHERE deleted_at = 0;

// Delete
UPDATE users SET deleted_at = /* current unix second */ WHERE ID = 1;

你也可以指定使用毫秒（milli）或纳秒nano 作为值，例如:

type User struct {
  ID    uint
  Name  string
  DeletedAt soft_delete.DeletedAt `gorm:"softDelete:milli"`
  // DeletedAt soft_delete.DeletedAt `gorm:"softDelete:nano"`
}

// Query
SELECT * FROM users WHERE deleted_at = 0;

// Delete
UPDATE users SET deleted_at = /* current unix milli second or nano second */ WHERE ID = 1;

Use 1 / 0 AS Delete Flag

import "gorm.io/plugin/soft_delete"

type User struct {
  ID    uint
  Name  string
  IsDel soft_delete.DeletedAt `gorm:"softDelete:flag"`
}

// Query
SELECT * FROM users WHERE is_del = 0;

// Delete
UPDATE users SET is_del = 1 WHERE ID = 1;

混合模式可以使用0、1或 unix秒 来标记数据是否已删除，同时保存已删除的时间。

type User struct {
  ID        uint
  Name      string
  DeletedAt time.Time
  IsDel     soft_delete.DeletedAt `gorm:"softDelete:flag,DeletedAtField:DeletedAt"` // use `1` `0`
  // IsDel     soft_delete.DeletedAt `gorm:"softDelete:,DeletedAtField:DeletedAt"` // use `unix second`
  // IsDel     soft_delete.DeletedAt `gorm:"softDelete:nano,DeletedAtField:DeletedAt"` // use `unix nano second`
}

// Query
SELECT * FROM users WHERE is_del = 0;

// Delete
UPDATE users SET is_del = 1, deleted_at = /* current unix second */ WHERE ID = 1;

3、SQL 构建器

3.1 Raw SQL

使用Scan查询原始SQL

type Result struct {
  ID   int
  Name string
  Age  int
}

var result Result
db.Raw("SELECT id, name, age FROM users WHERE name = ?", 3).Scan(&result)

var age int
db.Raw("SELECT SUM(age) FROM users WHERE role = ?", "admin").Scan(&age)

var users []User
db.Raw("UPDATE users SET name = ? WHERE age = ? RETURNING id, name", "jinzhu", 20).Scan(&users)

3.2 Exec与原始SQL

db.Exec("DROP TABLE users")
db.Exec("UPDATE orders SET shipped_at = ? WHERE id IN ?", time.Now(), []int64{1, 2, 3})

// Exec with SQL Expression
db.Exec("UPDATE users SET money = ? WHERE name = ?", gorm.Expr("money * ? + ?", 10000, 1), "jinzhu")

GORM允许缓存准备语句以提高性能，详细信息请检查performance

3.3 命名参数

GORM支持命名参数。sql.NamedArg，map[string]interface{}{} 和struct，例如:

db.Where("name1 = @name OR name2 = @name", sql.Named("name", "jinzhu")).Find(&user)
// SELECT * FROM `users` WHERE name1 = "jinzhu" OR name2 = "jinzhu"

db.Where("name1 = @name OR name2 = @name", map[string]interface{}{"name": "jinzhu2"}).First(&result3)
// SELECT * FROM `users` WHERE name1 = "jinzhu2" OR name2 = "jinzhu2" ORDER BY `users`.`id` LIMIT 1

// Named Argument with Raw SQL
db.Raw("SELECT * FROM users WHERE name1 = @name OR name2 = @name2 OR name3 = @name",
   sql.Named("name", "jinzhu1"), sql.Named("name2", "jinzhu2")).Find(&user)
// SELECT * FROM users WHERE name1 = "jinzhu1" OR name2 = "jinzhu2" OR name3 = "jinzhu1"

db.Exec("UPDATE users SET name1 = @name, name2 = @name2, name3 = @name",
   sql.Named("name", "jinzhunew"), sql.Named("name2", "jinzhunew2"))
// UPDATE users SET name1 = "jinzhunew", name2 = "jinzhunew2", name3 = "jinzhunew"

db.Raw("SELECT * FROM users WHERE (name1 = @name AND name3 = @name) AND name2 = @name2",
   map[string]interface{}{"name": "jinzhu", "name2": "jinzhu2"}).Find(&user)
// SELECT * FROM users WHERE (name1 = "jinzhu" AND name3 = "jinzhu") AND name2 = "jinzhu2"

type NamedArgument struct {
  Name string
  Name2 string
}

db.Raw("SELECT * FROM users WHERE (name1 = @Name AND name3 = @Name) AND name2 = @Name2",
   NamedArgument{Name: "jinzhu", Name2: "jinzhu2"}).Find(&user)
// SELECT * FROM users WHERE (name1 = "jinzhu" AND name3 = "jinzhu") AND name2 = "jinzhu2"

3.4 DryRun模式

生成SQL及其参数，但不执行，可用于准备或测试生成的SQL，查看会话了解详细信息

stmt := db.Session(&Session{DryRun: true}).First(&user, 1).Statement
stmt.SQL.String() //=> SELECT * FROM `users` WHERE `id` = $1 ORDER BY `id`
stmt.Vars         //=> []interface{}{1}

3.5 ToSQL

不执行，返回生成的SQL。

GORM使用database/sql的参数占位符来构造sql语句，它会自动转义参数以避免sql注入，但生成的sql不提供安全保证，请仅用于调试。

sql := DB.ToSQL(func(tx *gorm.DB) *gorm.DB {
  return tx.Model(&User{}).Where("id = ?", 100).Limit(10).Order("age desc").Find(&[]User{})
})
sql //=> SELECT * FROM "users" WHERE id = 100 AND "users"."deleted_at" IS NULL ORDER BY age desc LIMIT 10

3.6 Row & Rows

获取结果为*sql.Row

// Use GORM API build SQL
row := db.Table("users").Where("name = ?", "jinzhu").Select("name", "age").Row()
row.Scan(&name, &age)

// Use Raw SQL
row := db.Raw("select name, age, email from users where name = ?", "jinzhu").Row()
row.Scan(&name, &age, &email)

获取结果为*sql.Rows

// Use GORM API build SQL
rows, err := db.Model(&User{}).Where("name = ?", "jinzhu").Select("name, age, email").Rows()
defer rows.Close()
for rows.Next() {
  rows.Scan(&name, &age, &email)

  // do something
}

// Raw SQL
rows, err := db.Raw("select name, age, email from users where name = ?", "jinzhu").Rows()
defer rows.Close()
for rows.Next() {
  rows.Scan(&name, &age, &email)

  // do something
}

关于如何批量查询和处理记录，请查看FindInBatches
如何构建复杂的SQL查询, 请查看组条件

3.7 扫描*sql.Rows进入 struct

使用ScanRows将一行扫描到一个结构体中，例如:

rows, err := db.Model(&User{}).Where("name = ?", "jinzhu").Select("name, age, email").Rows() // (*sql.Rows, error)
defer rows.Close()

var user User
for rows.Next() {
  // ScanRows scan a row into user
  db.ScanRows(rows, &user)

  // do something
}

3.8 Connection

在同一个db tcp连接中运行多个SQL(不是在一个事务中)

db.Connection(func(tx *gorm.DB) error {
  tx.Exec("SET my.role = ?", "admin")

  tx.First(&User{})
})

3.9 进阶

3.9.1 子句 (Clauses)

GORM使用SQL构建器在内部生成SQL，对于每个操作，GORM创建一个*gorm.Statement对象，所有GORM api为Statement添加/更改子句(Clause)，最后GORM根据这些子句生成SQL

例如，当使用First进行查询时，它会将以下子句添加到Statement中

clause.Select{Columns: "*"}
clause.From{Tables: clause.CurrentTable}
clause.Limit{Limit: 1}
clause.OrderByColumn{
  Column: clause.Column{Table: clause.CurrentTable, Name: clause.PrimaryKey},
}

然后GORM构建最后在Query 回调中查询SQL，如下:

Statement.Build("SELECT", "FROM", "WHERE", "GROUP BY", "ORDER BY", "LIMIT", "FOR")

会生成SQL:

SELECT * FROM `users` ORDER BY `users`.`id` LIMIT 1

你可以定义你自己的子句（Clause ），并与GORM一起使用，它需要实现接口

查看示例以供参考

3.9.2 Clause Builder

对于不同的数据库，子句可能生成不同的SQL，例如:

db.Offset(10).Limit(5).Find(&users)
// Generated for SQL Server
// SELECT * FROM "users" OFFSET 10 ROW FETCH NEXT 5 ROWS ONLY
// Generated for MySQL
// SELECT * FROM `users` LIMIT 5 OFFSET 10

支持这样是因为GORM允许数据库驱动程序注册子句构建器替换默认的子句生成器，以Limit为例

3.9.3 Clause Options

GORM定义了许多子句，一些子句提供了可用于您的应用程序高级选项

虽然他们中的大多数很少使用，如果你发现GORM公共API不能满足你的需求，可能会很好地检查他们，例如:

db.Clauses(clause.Insert{Modifier: "IGNORE"}).Create(&user)
// INSERT IGNORE INTO users (name,age...) VALUES ("jinzhu",18...);

3.9.4 StatementModifier

GORM提供了StatementModifier接口，允许您修改语句以匹配您的需求，以Hints为例

import "gorm.io/hints"

db.Clauses(hints.New("hint")).Find(&User{})
// SELECT * /*+ hint */ FROM `users`