作用域允许你复用通用的逻辑，这种共享逻辑需要定义为类型func(*gorm.DB) *gorm.DB。

查询

Scope 查询示例：

func AmountGreaterThan1000(db *gorm.DB) *gorm.DB {
  return db.Where("amount > ?", 1000)
}

func PaidWithCreditCard(db *gorm.DB) *gorm.DB {
  return db.Where("pay_mode_sign = ?", "C")
}

func PaidWithCod(db *gorm.DB) *gorm.DB {
  return db.Where("pay_mode_sign = ?", "C")
}

func OrderStatus(status []string) func (db *gorm.DB) *gorm.DB {
  return func (db *gorm.DB) *gorm.DB {
    return db.Where("status IN (?)", status)
  }
}

db.Scopes(AmountGreaterThan1000, PaidWithCreditCard).Find(&orders)
// 查找所有金额大于 1000 的信用卡订单

db.Scopes(AmountGreaterThan1000, PaidWithCod).Find(&orders)
// 查找所有金额大于 1000 的 COD 订单

db.Scopes(AmountGreaterThan1000, OrderStatus([]string{"paid", "shipped"})).Find(&orders)
// 查找所有金额大于1000 的已付款或已发货订单

分页

func Paginate(r *http.Request) func(db *gorm.DB) *gorm.DB {
  return func (db *gorm.DB) *gorm.DB {
    q := r.URL.Query()
    page, _ := strconv.Atoi(q.Get("page"))
    if page <= 0 {
      page = 1
    }

    pageSize, _ := strconv.Atoi(q.Get("page_size"))
    switch {
    case pageSize > 100:
      pageSize = 100
    case pageSize <= 0:
      pageSize = 10
    }

    offset := (page - 1) * pageSize
    return db.Offset(offset).Limit(pageSize)
  }
}

db.Scopes(Paginate(r)).Find(&users)
db.Scopes(Paginate(r)).Find(&articles)

动态表

使用 Scopes 来动态指定查询的表

func TableOfYear(user *User, year int) func(db *gorm.DB) *gorm.DB {
  return func(db *gorm.DB) *gorm.DB {
        tableName := user.TableName() + strconv.Itoa(year)
        return db.Table(tableName)
  }
}

DB.Scopes(TableOfYear(user, 2019)).Find(&users)
// SELECT * FROM users_2019;

DB.Scopes(TableOfYear(user, 2020)).Find(&users)
// SELECT * FROM users_2020;

// Table form different database
func TableOfOrg(user *User, dbName string) func(db *gorm.DB) *gorm.DB {
  return func(db *gorm.DB) *gorm.DB {
        tableName := dbName + "." + user.TableName()
        return db.Table(tableName)
  }
}

DB.Scopes(TableOfOrg(user, "org1")).Find(&users)
// SELECT * FROM org1.users;

DB.Scopes(TableOfOrg(user, "org2")).Find(&users)
// SELECT * FROM org2.users;

更新

Scope 更新、删除示例：

func CurOrganization(r *http.Request) func(db *gorm.DB) *gorm.DB {
  return func (db *gorm.DB) *gorm.DB {
    org := r.Query("org")

    if org != "" {
      var organization Organization
      if db.Session(&Session{}).First(&organization, "name = ?", org).Error == nil {
        return db.Where("org_id = ?", organization.ID)
      }
    }

    db.AddError("invalid organization")
    return db
  }
}

db.Model(&article).Scopes(CurOrganization(r)).Update("Name", "name 1")
// UPDATE articles SET name = "name 1" WHERE org_id = 111
db.Scopes(CurOrganization(r)).Delete(&Article{})
// DELETE FROM articles WHERE org_id = 111