Golang: Gorm Error on insert or update on table violates foreign key contraint

Question

Im trying to create a REST service using Gorm that on startup will drop the database tables from my Postgres DB and then create and populate them with test data. The entities I am using are as follows:

type Group struct {
    ID       uuid.UUID `gorm:"PrimaryKey" json:"id"`
    Name     string     `json:"name"`
    Sessions []Session `gorm:"foreignKey:ID" json:"sessions"`
}

type Session struct {
    ID      uuid.UUID `gorm:"PrimaryKey"`
    GroupID uuid.UUID `gorm:"foreignKey:ID"`
    Name    string
    From    time.Time
    To      time.Time
}

type Player struct {
    ID       uuid.UUID `gorm:"PrimaryKey" json:"id"`
    Sessions []Session `gorm:"many2many:players_sessions" json:"sessions"`
    Groups   []Group   `gorm:"many2many:players_groups" json:"groups"`
    Username string    `gorm:"type:varchar;NOT NULL" json:"username"`
    Cookie   string    `json:"cookie"`
}

On startup, when the tables have been dropped they are recreated and populated using this code:

func PopulateDB(db *gorm.DB) {
    db.AutoMigrate(&entities.Group{}, &entities.Player{}, &entities.Session{})

    players := []entities.Player{
        {ID: uuid.New(), Username: "Player 1", Cookie: ""},
        {ID: uuid.New(), Username: "Player 2", Cookie: ""},
        {ID: uuid.New(), Username: "Player 3", Cookie: ""},
    }
    for index := range players {
        db.Create(&players[index])
    }

    group := entities.Group{ID: uuid.New(), Name: "My Group"}
    db.Create(&group)

    sessions := []entities.Session{
        {
            ID: uuid.New(), 
            GroupID: group.ID,
            Name: "Session 1", 
            From: time.Now(), 
            To: time.Now().Add(12 * time.Hour),
        },
        {
            ID:      uuid.New(),
            GroupID: group.ID,
            Name:    "Session 2",
            From:    time.Now().Add(24 * time.Hour),
            To:      time.Now().Add(36 * time.Hour),
        },
        
    }
    for index := range sessions {
        db.Model(&group).Association("Sessions").Append(&sessions[index])
        // db.Create(&sessions[index])
    }

    // Make player >-< groups connections
    for index := range players {
        db.Model(&players[index]).Association("Groups").Append(&group)
    }

    // Make player >-< session connections
    for index := range players {
        db.Model(&players[index]).Association("Sessions").Append(&sessions[0])
        if index%2 == 0 {
            db.Model(&players[index]).Association("Sessions").Append(&sessions[1])
        }
    }
}

The issue I am having is that when the test data is inserted it returns the following error: ERROR: insert or update on table "sessions" violates foreign key constraint "fk_groups_sessions" (SQLSTATE 23503). This occurs on the line db.Model(&group).Association("Sessions").Append(&sessions[index]).

I'm under the impression that this is being caused by trying to insert a Session without a valid Group foreign key, but I'm not knowledgeable enough with Gorm to understand what I'm doing wrong. Any help would be much appreciated!

Answer 1

type Group struct {
    ID       uuid.UUID `gorm:"PrimaryKey" json:"id"`
    Name     string     `json:"name"`
    Sessions []Session `gorm:"foreignKey:GroupID" json:"sessions"
}

type Session struct {
    ID      uuid.UUID `gorm:"PrimaryKey"`
    GroupID uuid.UUID `gorm:"foreignKey:ID"`
    Name    string
    From    time.Time
    To      time.Time
}

This should fix it, the reference is changed from ID to GroupID

Answer 2

If this is your database schema design, it should be like this.

type Group struct {
    ID       uuid.UUID `gorm:"primaryKey"`
    Name     string    `gorm:"column:name"`
}

type Session struct {
    ID      uuid.UUID `gorm:"primaryKey"`
    GroupID uuid.UUID `gorm:"column:group_id"
    Group   Group     `gorm:"foreignKey:ID"`
    Name    string    `gorm:"column:name"`
    From    time.Time `gorm:"column:from"`
    To      time.Time `gorm:"column:to"`
}

type Player struct {
    ID        uuid.UUID `gorm:"primaryKey"`
    Username  string    `gorm:"column:username;type:varchar;NOT NULL"`
    Cookie    string    `gorm:"column:cookie"`
    SessionID uuid.UUID `gorm:"column:session_id"`
    GroupID   uuid.UUID `gorm:"column:group_id"`
    Sessions  []Session `gorm:"many2many:players_sessions;foreignKey:SessionID;references:ID;"`
    Groups    []Group   `gorm:"many2many:players_groups;foreignKey:GroupID;references:ID;"`
}