Golang Gorm reverse hasMany relation - go

Question

We can Easy to get the child of hasMany relation in Golang Gorm with Preload.
But how to get reverse of relation.
type Owner struct {
ID int `gorm:"column:id" json:"id"`
Name string `gorm:"column:name" json:"name"`
Projects []Project `gorm:"foreignkey:OwnerID" json:"projects"`
}
type Project struct {
ID int `gorm:"column:id" json:"id"`
Name string `gorm:"column:name" json:"name"`
OwnerID int `gorm:"column:owner_id" json:"owner_id"`
Gallery []Gallery `gorm:"foreignkey:ProjectID" json:"gallery"`
}
type Gallery struct {
ID int `gorm:"column:id" json:"id"`
ProjectID int `gorm:"column:project_id" json:"project_id"`
Url string `gorm:"column:url" json:"url"`
Title string `gorm:"column:title" json:"title"`
Description string `gorm:"column:description" json:"description"`
}
we can populate Gallery inside Project with Preload, like this:
db.Preload("Gallery").Find(&project)
How to get reverse, we want load project from gallery or owner from project?
the result I want some thing like this, when get project in form of json:
{
"id": 1,
"name": "Name Of Project",
"owner": {
"id":1,
"name": "Owner 1"
},
"gallery":[]
}

Answer 1

I am a bit late to the game with this answer, but you can define inverse relationships (belongs to) on the child models by doing this:
type ParentModel struct {
Children []Child // This is a 'has many'
}
type Child struct {
ParentModelID int // These behave as a 'belongs to'
ParentModel ParentModel // You need both the model and the modelID
}
Full working example
This uses the models from your question. A note on performance, the preloading in gorm isn't done via joins. It's done via additional SQL queries, the more nesting the more queries. If you have commonly executed preloading, it could be worth writing a raw query and a customer scanner to use an optimised query.
package main
import (
"fmt"
"gorm.io/driver/sqlite"
"gorm.io/gorm"
)
type Owner struct {
gorm.Model
Projects []Project
}
type Project struct {
gorm.Model
OwnerID int
Owner Owner
Gallery []Gallery
}
type Gallery struct {
gorm.Model
ProjectID int
Project Project
}
func main() {
db, err := gorm.Open(sqlite.Open("many2many.db"), &gorm.Config{})
if err != nil {
panic("failed to connect database")
}
err = db.AutoMigrate(&Owner{}, &Project{}, &Gallery{})
if err != nil {
return
}
ownerOne := Owner{}
db.Create(&ownerOne)
projectOne := Project{Owner: ownerOne}
projectTwo := Project{Owner: ownerOne}
db.Create(&projectOne)
db.Create(&projectTwo)
galleryOne := Gallery{Project: projectOne}
galleryTwo := Gallery{Project: projectOne}
galleryThree := Gallery{Project: projectTwo}
galleryFour := Gallery{Project: projectTwo}
db.Create(&galleryOne)
db.Create(&galleryTwo)
db.Create(&galleryThree)
db.Create(&galleryFour)
// Find by project and preload owners
fetchedProject := Project{}
db.Preload("Owner").Find(&fetchedProject, projectOne.ID)
fmt.Println(fetchedProject.Owner)
// Find by a gallery and preload project and owner
fetchedGallery := Gallery{}
db.Preload("Project.Owner").Find(&fetchedGallery, galleryOne.ID)
fmt.Printf("Gallery.id = %d --> Project.id = %d --> Owner.id = %d\n", fetchedGallery.ID,
fetchedGallery.Project.ID, fetchedGallery.Project.Owner.ID)
}