Go gorm check a model is associated with another model - go

Question

I have a race model
type Race struct {
gorm.Model
Title string
Date string
Token string
Heats []Heat `gorm:"constraint:OnUpdate:CASCADE,OnDelete:CASCADE;"`
Runners []Runner `gorm:"many2many:race_runners;constraint:OnUpdate:CASCADE,OnDelete:SET NULL;"`
}
I want to add runners into heats but before doing so I want to assert that the runner actually is in the race
var race models.Race
if err := models.DB.Preload("Runners").Find(&race, "runners.id IN ?", []uint{runner.ID}).Error; err != nil {
c.JSON(http.StatusNotFound, gin.H{"error": "Runner not in race!"})
return
}
I'm getting the following error
near "?": syntax error
Because I'm having a many to many relationship between Race and Runners I doubt that I can just do runners.id in the condition I pass to my Find method. But I'm not sure how to actually do what I want to achieve. Which is ensure the runner is already in the race before adding it to a heat. Any suggestions?

Answer 1

The where query wants to be in the Preload part. Then you can check the len(fetchedRace.Runners) < 1, and if that is true, the runner is not in that race.
fetchedRace := Race{}
db.Preload("Runners", "runners.id = ?", runnerOne.ID).Find(&fetchedRace, race.ID)
Full working example
package main
import (
"fmt"
"gorm.io/driver/sqlite"
"gorm.io/gorm"
)
type Race struct {
gorm.Model
Title string
Date string
Token string
Runners []Runner `gorm:"many2many:race_runners;constraint:OnUpdate:CASCADE,OnDelete:SET NULL;"`
}
type Runner struct {
gorm.Model
Name string
}
func main() {
db, err := gorm.Open(sqlite.Open("many2many.db"), &gorm.Config{})
if err != nil {
panic("failed to connect database")
}
// Migrate the schema
_ = db.AutoMigrate(&Race{}, &Runner{})
raceOne := Race{
Title: "Race One",
}
db.Create(&raceOne)
runnerOne := Runner{
Name: "Runner One",
}
runnerTwo := Runner{
Name: "Runner Two",
}
db.Create(&runnerOne)
db.Create(&runnerTwo)
// Associate runners with race
err = db.Model(&raceOne).Association("Runners").Append([]Runner{runnerOne, runnerTwo})
// Fetch from DB
fetchedRace := Race{}
db.Debug().Preload("Runners", "runners.id = ?", runnerOne.ID).Find(&fetchedRace, raceOne.ID)
if len(fetchedRace.Runners) < 1 {
fmt.Println("error, runner not in race")
}
db.Delete(&raceOne)
db.Delete(&runnerOne)
db.Delete(&runnerTwo)
}
Using a custom SQL statement
This can be done in one SQL statement and gorm isn't very optimal with it's preloading, so if you are planning on using this often I would probably do something like this:
func isRunnerInRace(db *gorm.DB, runnerId uint, raceId uint) bool {
count := 0
db.Raw("SELECT COUNT(runner_id) FROM race_runners WHERE race_id = ? AND runner_id = ?", raceId,
runnerId).Scan(&count)
return count > 0
}