gorm Preload与Joins带条件查询--闭坑指南与示例

一. Preload与Joins的区别
二. 使用示例
- 1. Preload
- - 1.1 Preload 带条件查询
- 2 Joins
- - 2.1 Joins 带条件查询
  - 2.2 Joins 模糊查询
  - 2.3 使Joins支持一对多关系的查询
  - 2.4 Joins方法的错误示范

一. Preload与Joins的区别

Preload方法是用来加载关联字段（belongTo、many2many、hasOne、hasMany）的数据的。
gorm中的Joins方法仅适用的查询，无法加载关联字段内容。并且，gorm原生的方法只支持一对一关系(has one, belongs to)。

二. 使用示例

示例中用到的结构体如下

type Child struct {gorm.ModelChildName string `gorm:"column:child_name;unique"`Toys      []Toy  `gorm:"foreignKey:ChildId"`
}
func (Child) TableName() string {return "child"
}type Toy struct {gorm.ModelName    string `gorm:"column:name"`ChildId uint   `gorm:"column:child_id"`Child   *Child `gorm:"foreignKey:ChildId"`
}
func (Toy) TableName() string {return "toy"
}

并生成以下数据

var children = []Child{{ChildName: "刘涛",Toys: []Toy{{Name: "纸飞机"},{Name: "小火车"},},},{ChildName: "王斌",Toys: []Toy{{Name: "玩具兵"},},},
}
db.Create(&children)

1. Preload

1.1 Preload 带条件查询

举个例子：有个活动需要带所有孩子都参加，但是每个孩子都只能带名为纸飞机的玩具。
使用如下查询方法：

var children []Child
db.Preload("Toys", db.Where(&Toy{Name: "纸飞机"})).Find(&children)
//也可以写成
//db.Preload("Toys", "name = ?", "纸飞机").Find(&children)

等同于sql语句

SELECT * FROM "toy" WHERE "toy"."child_id" IN (1,2) AND name =  '纸飞机' AND "toy"."deleted_at" IS NULL;
SELECT * FROM "child" WHERE "child"."deleted_at" IS NULL;

输出结果转为json(为了易于查看，省略了一些不必要的字段):

[{"ID": 1,"ChildName": "刘涛","Toys": [{"ID": 1,"ChildId": 1,"Name": "纸飞机"}]},{"ID": 2,"ChildName": "王斌","Toys": []}
]

可以看到，王斌小朋友也出现在了查询结果中，但是他的"Toys”是空的。因为你的查询条件只允许小朋友们带纸飞机。

2 Joins

2.1 Joins 带条件查询

因为joins只支持has one和 belongs to，所有这里的例子换个方向：查询哪些玩具是属于刘涛的。
查询示例如下:

var toys []Toy
//注意where中的表别名一定要加双引号
db.Joins("Child").Where("\"Child\".child_name = ?", "刘涛").Find(&toys)//也可以写成这样，这里是不需要双引号的
db.Debug().Joins("Child").Clauses(clause.Eq{Column: "Child.child_name",Value:  "刘涛",
}).Find(&toys)

等同于sql:

SELECT "toy"."id","toy"."created_at","toy"."updated_at","toy"."deleted_at","toy"."child_id","toy"."name","Child"."id" AS "Child__id","Child"."created_at" AS "Child__created_at","Child"."updated_at" AS "Child__updated_at","Child"."deleted_at" AS "Child__deleted_at","Child"."child_name" AS "Child__child_name" FROM "toy" LEFT JOIN "child" "Child" ON "toy"."child_id" = "Child"."id" AND "Child"."deleted_at" IS NULL WHERE "Child".child_name = '刘涛' AND "toy"."deleted_at" IS NULL

主要是这断话：LEFT JOIN "child" "Child"，gorm自动給表起了一个别名Child。

查询结果:

[{"ID": 1,"CreatedAt": "2022-11-09T19:51:38.008533+08:00","UpdatedAt": "2022-11-09T19:51:38.008533+08:00","DeletedAt": null,"ChildId": 1,"Child": {"ID": 1,"CreatedAt": "2022-11-09T19:51:38.006354+08:00","UpdatedAt": "2022-11-09T19:51:38.006354+08:00","DeletedAt": null,"ChildName": "刘涛","Toys": null},"Name": "纸飞机"},{"ID": 2,"CreatedAt": "2022-11-09T19:51:38.008533+08:00","UpdatedAt": "2022-11-09T19:51:38.008533+08:00","DeletedAt": null,"ChildId": 1,"Child": {"ID": 1,"CreatedAt": "2022-11-09T19:51:38.006354+08:00","UpdatedAt": "2022-11-09T19:51:38.006354+08:00","DeletedAt": null,"ChildName": "刘涛","Toys": null},"Name": "小火车"}
]

2.2 Joins 模糊查询

模糊查询最省力的方法是使用子句构造器,其它类似的方法就不展示了:

//查询拥有‘汽车’玩具的孩子们
db = db.Joins("Toys").Clauses(clause.Like{Column: "Toys.name",Value:  "%" + "汽车" + "%",}).Find(&children)

2.3 使Joins支持一对多关系的查询

这个方法其实也支持多对多关系，和sql语法是原理是一样的。
例子：查询有纸飞机玩具的孩子们，没有的孩子不展示。

var children []Child
db.Table(fmt.Sprintf("%v AS t1", Child{}.TableName())).Joins(fmt.Sprintf("LEFT JOIN %v AS t2 on t1.id =t2.child_id ", Toy{}.TableName())).Where("t2.name = ?", "纸飞机").Find(&children)

等同于sql语句

SELECT "t1"."id","t1"."created_at","t1"."updated_at","t1"."deleted_at","t1"."child_name" FROM child AS t1 left join toy t2 on t1.id =t2.child_id  WHERE t2.name = '纸飞机' AND "t1"."deleted_at" IS NULL

查询结果

[{"ID": 1,"CreatedAt": "2022-11-09T19:51:38.006354+08:00","UpdatedAt": "2022-11-09T19:51:38.006354+08:00","DeletedAt": null,"ChildName": "刘涛","Toys": null}
]

可以看到，查询结果没问题，但是Toys是空的。加一个Preload方法就行了

db.Preload("Toys").Table(fmt.Sprintf("%v AS t1", Child{}.TableName())).Joins(fmt.Sprintf("LEFT JOIN %v AS t2 on t1.id =t2.child_id ", Toy{}.TableName())).Where("t2.name = ?", "纸飞机").Find(&children)

输出结果

[{"ID": 1,"CreatedAt": "2022-11-09T19:51:38.006354+08:00","UpdatedAt": "2022-11-09T19:51:38.006354+08:00","DeletedAt": null,"ChildName": "刘涛","Toys": [{"ID": 1,"CreatedAt": "2022-11-09T19:51:38.008533+08:00","UpdatedAt": "2022-11-09T19:51:38.008533+08:00","DeletedAt": null,"ChildId": 1,"Child": null,"Name": "纸飞机"},{"ID": 2,"CreatedAt": "2022-11-09T19:51:38.008533+08:00","UpdatedAt": "2022-11-09T19:51:38.008533+08:00","DeletedAt": null,"ChildId": 1,"Child": null,"Name": "小火车"}]}
]

2.4 Joins方法的错误示范

需要注意的是，Joins方法的查询条件需要放在Where方法中。
但是很多人会和Preload方法混淆，写成这样

//错误示例1
db.Joins("Child", db.Where(&Child{ChildName: "刘涛"})).Find(&toys)
//错误示例2
db.Joins("Child", "Child.child_name = '刘涛'").Find(&toys)

错误示例1 查询出来的玩具兵也变成刘涛的了，完全不对；
官方文档中虽然有介绍到这个方法，但是实际使用中意义不明，而且还存在bug。

错误示例2 完全无意义的查询方法。

[{"ID": 1,"CreatedAt": "2022-11-09T19:51:38.008533+08:00","UpdatedAt": "2022-11-09T19:51:38.008533+08:00","DeletedAt": null,"ChildId": 1,"Child": {"ID": 1,"CreatedAt": "2022-11-09T19:51:38.006354+08:00","UpdatedAt": "2022-11-09T19:51:38.006354+08:00","DeletedAt": null,"ChildName": "刘涛","Toys": null},"Name": "纸飞机"},{"ID": 2,"CreatedAt": "2022-11-09T19:51:38.008533+08:00","UpdatedAt": "2022-11-09T19:51:38.008533+08:00","DeletedAt": null,"ChildId": 1,"Child": {"ID": 1,"CreatedAt": "2022-11-09T19:51:38.006354+08:00","UpdatedAt": "2022-11-09T19:51:38.006354+08:00","DeletedAt": null,"ChildName": "刘涛","Toys": null},"Name": "小火车"},{"ID": 3,"CreatedAt": "2022-11-09T19:51:38.008533+08:00","UpdatedAt": "2022-11-09T19:51:38.008533+08:00","DeletedAt": null,"ChildId": 2,"Child": {"ID": 2,"CreatedAt": "2022-11-09T19:51:38.006354+08:00","UpdatedAt": "2022-11-09T19:51:38.006354+08:00","DeletedAt": null,"ChildName": "王斌","Toys": null},"Name": "玩具兵"}
]

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gorm Preload与Joins带条件查询--闭坑指南与示例

gorm Preload与Joins带条件查询--闭坑指南与示例

一. Preload与Joins的区别

二. 使用示例

1. Preload

1.1 Preload 带条件查询

2 Joins

2.1 Joins 带条件查询

2.2 Joins 模糊查询

2.3 使Joins支持一对多关系的查询

2.4 Joins方法的错误示范

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