Gorm 链式操作

GORM 允许进行链式操作，所以我们可以像这样写代码

db.Where("name = ?", "jinzhu").Where("age = ?", 18).First(&user)

GORM 中有三种类型的方法： 链式方法 、终结方法 、新建会话方法

在 链式方法 , 终结方法 之后, GORM 返回一个初始化的 *gorm.DB 实例，实例不能安全地重复使用，并且新生成的 SQL 可能会被先前的条件污染，例如：

queryDB := DB.Where("name = ?", "jinzhu")

queryDB.Where("age > ?", 10).First(&user)
// SELECT * FROM users WHERE name = "jinzhu" AND age > 10

queryDB.Where("age > ?", 20).First(&user2)
// SELECT * FROM users WHERE name = "jinzhu" AND age > 10 AND age > 20

为了重新使用初始化的 *gorm.DB 实例, 我们可以使用 新建会话方法 创建一个可共享的 *gorm.DB, 例如:

queryDB := DB.Where("name = ?", "jinzhu").Session(&gorm.Session{})

queryDB.Where("age > ?", 10).First(&user)
// SELECT * FROM users WHERE name = "jinzhu" AND age > 10

queryDB.Where("age > ?", 20).First(&user2)
// SELECT * FROM users WHERE name = "jinzhu" AND age > 20

链式方法

链式方法是将 Clauses 修改或添加到当前 Statement 的方法，例如：

Where, Select, Omit, Joins, Scopes, Preload, Raw (Raw can’t be used with other chainable methods to build SQL)…

这是 完整方法列表，也可以查看 SQL 构建器 获取更多关于 Clauses 的信息

终结方法

终结（方法） 是会立即执行注册回调的方法，然后生成并执行 SQL，比如这些方法：

• Create
• First
• Find
• Take
• Save
• Update
• Delete
• Scan
• Row
• Rows ...

查看完整方法列表

新建会话方法

GORM 定义了 Session 、WithContext 、Debug 方法做为 新建会话方法，查看会话 获取详情.

在 链式方法, Finisher 方法之后, GORM 返回一个初始化的 *gorm.DB 实例，不能安全地再使用。我们应该使用 新建会话方法 来标记 *gorm.DB 为可共享。

让我们用实例来解释它：

示例 1：

db, err := gorm.Open(sqlite.Open("test.db"), &gorm.Config{})
// db is a new initialized `*gorm.DB`, which is safe to reuse

db.Where("name = ?", "jinzhu").Where("age = ?", 18).Find(&users)
// `Where("name = ?", "jinzhu")` is the first chain method call, it will create an initialized `*gorm.DB` instance, aka `*gorm.Statement`
// `Where("age = ?", 18)` is the second chain method call, it reuses the above `*gorm.Statement`, adds new condition `age = 18` to it
// `Find(&users)` is a finisher method, it executes registered Query Callbacks, which generates and runs the following SQL:
// SELECT * FROM users WHERE name = 'jinzhu' AND age = 18;

db.Where("name = ?", "jinzhu2").Where("age = ?", 20).Find(&users)
// `Where("name = ?", "jinzhu2")` is also the first chain method call, it creates a new `*gorm.Statement`
// `Where("age = ?", 20)` reuses the above `Statement`, and add conditions to it
// `Find(&users)` is a finisher method, it executes registered Query Callbacks, generates and runs the following SQL:
// SELECT * FROM users WHERE name = 'jinzhu2' AND age = 20;

db.Find(&users)
// `Find(&users)` is a finisher method call, it also creates a new `Statement` and executes registered Query Callbacks, generates and runs the following SQL:
// SELECT * FROM users;

(错误的) 示例2：

db, err := gorm.Open(sqlite.Open("test.db"), &gorm.Config{})
// db is a new initialized *gorm.DB, which is safe to reuse

tx := db.Where("name = ?", "jinzhu")
// `Where("name = ?", "jinzhu")` returns an initialized `*gorm.Statement` instance after chain method `Where`, which is NOT safe to reuse

// good case
tx.Where("age = ?", 18).Find(&users)
// `tx.Where("age = ?", 18)` use the above `*gorm.Statement`, adds new condition to it
// `Find(&users)` is a finisher method call, it executes registered Query Callbacks, generates and runs the following SQL:
// SELECT * FROM users WHERE name = 'jinzhu' AND age = 18

// bad case
tx.Where("age = ?", 28).Find(&users)
// `tx.Where("age = ?", 18)` also use the above `*gorm.Statement`, and keep adding conditions to it
// So the following generated SQL is polluted by the previous conditions:
// SELECT * FROM users WHERE name = 'jinzhu' AND age = 18 AND age = 28;

示例 3：

db, err := gorm.Open(sqlite.Open("test.db"), &gorm.Config{})
// db is a new initialized *gorm.DB, which is safe to reuse

tx := db.Where("name = ?", "jinzhu").Session(&gorm.Session{})
tx := db.Where("name = ?", "jinzhu").WithContext(context.Background())
tx := db.Where("name = ?", "jinzhu").Debug()
// `Session`, `WithContext`, `Debug` returns `*gorm.DB` marked as safe to reuse, newly initialized `*gorm.Statement` based on it keeps current conditions

// good case
tx.Where("age = ?", 18).Find(&users)
// SELECT * FROM users WHERE name = 'jinzhu' AND age = 18

// good case
tx.Where("age = ?", 28).Find(&users)
// SELECT * FROM users WHERE name = 'jinzhu' AND age = 28;