Gorm 发现结构体字段未被识别

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我的数据库结构目标

one user can join many communities
and
one user can own a community

这是我想出的结构，正确吗？

type user struct {
    gorm.model
    communities []community `gorm:"many2many:user_communities" json:"communities"`
    firstname string `json:"firstname"`
    lastname string `json:"lastname"`
}

type community struct {
    gorm.model
    name string `json:"name"`
    userid uint `json:"userid"`
    users []user `gorm:"many2many:user_communities" json:"users"`
}

这是我的数据库设置

func connectmysql() {
    // connect to mysql database based on the environment variable
    dsn := os.getenv("mysql_dsn")

    database, err := gorm.open(mysql.open(dsn), &gorm.config{})

    if err != nil {
        panic("failed to connect to the database")
    }

    database.automigrate(&models.user{})
    database.automigrate(&models.community{})

    db = database
}

如何查询以检查用户是否属于社区？

到目前为止，我想出了这个，但它不起作用，可能我上面的模式是错误的

func CreateQuestion() {
    var community Community
    var foundUser User

    communityID = 1
    userID = 1

    // Find the community
    if err := database.Mysql.Where("id = ?", communityID).First(&community).Error; err != nil {
        c.JSON(http.StatusBadRequest, gin.H{"error": "Community does not exist"})
        return
    }
    
    // Im not sure whether this query is to check whether the user belongs to the community with id 1 or not.
    if err := database.Mysql.Preload("Community").Where("id = ?", userID).Find(&foundUser).Error; err != nil {
        c.JSON(http.StatusBadRequest, gin.H{"error": "user is not in the community"})
        return
    }
}

我遇到的错误

[错误] 结构发现无效字段 github.com/testing-be/community的字段users：定义一个有效的foreign 关系的关键或实现 valuer/scanner 接口

正确答案

您收到此错误的原因是 gorm 找不到定义的关系的外键。

将 id 字段添加到 user 和 community 结构中（或使用 gorm.model 而不是 id）。另外，还为 communities 字段添加 many2many。

type user struct {
    id uint `json:"id"`
    communities []community `gorm:"many2many:user_communities" json:"communities"`
    firstname string `json:"firstname"`
    lastname string `json:"lastname"`
}

type community struct {
    id uint `json:"id"`
    name string `json:"name"`
    userid uint `json:"userid"`
    users []user `gorm:"many2many:user_communities" json:"users"`
}

默认情况下，gorm 会尝试像这样解决关系：

// join table: user_communities
//   foreign key: user_id, reference: users.id
//   foreign key: community_id, reference: communities.id

如果 user_communities 表中的外键命名不同，则可以使用 here 进行处理。

关于preload函数，它接受字段名称，而不是字段类型，所以代码应该是这样的：

var founduser user
    if err := database.mysql.preload("communities").where("id = ?", userid).first(&founduser).error; err != nil {
        c.json(http.statusbadrequest, gin.h{"error": "user is not in the community"})
        return
    }

此代码加载与找到的用户有关系的所有社区。​​p>

编辑：

以下示例应帮助您确定特定用户是否属于特定社区：

//raw query
userid := 1
communityid := 1
var found sql.nullbool
if err := database.mysql.raw("select 1 from users u join user_communities uc on u.id = uc.user_id where u.id = ? and uc.community_id = ?", userid, communityid).scan(&found).error; err != nil {
      c.json(http.statusbadrequest, gin.h{"error": "error happened"})
            return
    }
if found.valid && found.bool {
  //user is part of the specified community
}

//with gorm functions
userid := 1
communityid := 1
var found sql.nullbool
if err := database.mysql.table("users u").joins("join user_communities uc on u.id = uc.user_id").where("u.id = ? and uc.community_id = ?", userid, communityid).select("1").scan(&found).error; err != nil {
      c.json(http.statusbadrequest, gin.h{"error": "error happened"})
            return
    }
if found.valid && found.bool {
  //user is part of the specified community
}

接下来，如何加载指定社区的指定用户的示例：

//finds just the specified user and loads just a specified community
userid := 1
communityid := 1
var user user
err := database.mysql.preload("communities", "id = ?", communityid).joins("join user_communities uc on u.id = uc.user_id").where("uc.community_id = ?", communityid).first(&user, userid).error
// err could be gorm.errornotfound, which means that a record is not found, so this needs to be checked before err != nil check

//finds just the specified user, loads all user's communities, but makes sure that a user belongs to a specific community
userid := 1
communityid := 1
var user user
err := database.mysql.preload("communities").joins("join user_communities uc on u.id = uc.user_id").where("uc.community_id = ?", communityid).first(&user, userid).error
// err could be gorm.errornotfound, which means that a record is not found, so this needs to be checked before err != nil check

也许您可能在结构中错过了 gorm.model ？

type User struct {
    gorm.Model
    Communities []Community `json:"communities"`
    FirstName string `json:"firstName"`
    LastName string `json:"lastName"`
}

type Community struct {
    gorm.Model
    Name string `json:"name"`
    UserID uint `json:"userID"`
    Users []User `gorm:"many2many:user_communities" json:"users"`
}

https://gorm.io/docs/many_to_many.html

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