Unsupported relations for schema with has many relation in GORM - go

Question

I have 2 models with Gorm like below:
type Post struct {
*gorm.Model
ID uint32 `gorm: "primarykey"`
PostTitle string `gorm:"posttitle;not null"`
PostSlug string `gorm:"postslug;unique;not null"`
PostCategoryID uint32 `gorm:"postcategoryid"`
PostCollections []PostCollection `gorm:"many2many:post_collection;"`
PostTags []PostTag `gorm:"many2many:post_tag; constraint:OnUpdate:CASCADE,OnDelete:SET NULL;"`
}
type PostCategory struct {
ID uint32 `gorm: "primarykey"`
PostCatName string `gorm:"postcatname;not null"`
PostCatSlug string `gorm:"postcatslug;unique;not null"`
PostCatParent uint32 `gorm: "postcatparent"`
Posts []Post `gorm:"foreignKey:PostCategoryID"`
}
I want get all posts of a parent category (which has many child categories) with function like below:
func GetPostByCategory(c *fiber.Ctx) error {
db := database.DB //connect database
var post []models.Post
var categories []models.PostCategory
cat := c.Params("catslug") //get slug value of parent category from route
cate := GetCategoryChild(cat) //get array of child category
db.Model(&categories).Where("id IN (?)", cate).Association("Posts").Find(&post)
return c.JSON(post)
}
It return nil array . Please help me to fix this

Answer 1

Problem
It is in your db.Model(&categories), you use an empty array of PostCategory. This will result in this query:
Code:
DB.Model(&categories).Where("id IN (?)", []int{1, 2, 3}).Association("Posts").Find(&post)
Output:
SELECT * FROM `posts` WHERE id IN (1,2,3) AND `posts`.`post_category_id` IN (NULL) AND `posts`.`deleted_at` IS NULL
The "id IN (?)", cate id is not the id of category, but the id of the post instead, because we are using Post type inside the .Find(&post)
You can try to apply logging into Gorm by using this config
databaseConfig := fmt.Sprintf("%s:%s#tcp(%s:%s)/%s?multiStatements=true&parseTime=true", "root", "", "127.0.0.1", "3306", "tester")
DB, _ = gorm.Open(mysql.Open(databaseConfig), &gorm.Config{
Logger: logger.Default.LogMode(logger.Info),
})
Solution
Try to insert some data inside your categories
This is My Full Code, I comment several line of your code because no full code was provided.
// https://stackoverflow.com/questions/69977516/unsupported-relations-for-schema-with-has-many-relation-in-gorm
package main
import (
"fmt"
"gorm.io/driver/mysql"
"gorm.io/gorm"
"gorm.io/gorm/logger"
)
type Post struct {
*gorm.Model
ID uint32 `gorm:"primarykey"`
PostTitle string `gorm:"posttitle;not null"`
PostSlug string `gorm:"postslug;unique;not null"`
PostCategoryID uint32 `gorm:"postcategoryid"`
// PostCollections []PostCollection `gorm:"many2many:post_collection;"`
// PostTags []PostTag `gorm:"many2many:post_tag; constraint:OnUpdate:CASCADE,OnDelete:SET NULL;"`
}
type PostCategory struct {
ID uint32 `gorm:"primarykey"`
PostCatName string `gorm:"postcatname;not null"`
PostCatSlug string `gorm:"postcatslug;unique;not null"`
PostCatParent uint32 `gorm:"postcatparent"`
Posts []Post `gorm:"foreignKey:PostCategoryID"`
}
var DB *gorm.DB
func main() {
databaseConfig := fmt.Sprintf("%s:%s#tcp(%s:%s)/%s?multiStatements=true&parseTime=true", "root", "", "127.0.0.1", "3306", "tester")
DB, _ = gorm.Open(mysql.Open(databaseConfig), &gorm.Config{
Logger: logger.Default.LogMode(logger.Info),
})
sqlDB, _ := DB.DB()
defer sqlDB.Close()
DB.AutoMigrate(&PostCategory{}, &Post{})
// ============ Function Get ================
var categories []PostCategory = []PostCategory{
{
ID: 1,
},
{
ID: 2,
},
}
var post []Post
DB.Model(&categories).Association("Posts").Find(&post)
fmt.Printf("%+v\n", post)
}
Output:
2021/11/23 20:28:41 D:/go/src/udemy-solving/00002/main.go:54
[1.054ms] [rows:3] SELECT * FROM `posts` WHERE id IN (1,2,3) AND `posts`.`post_category_id` IN (1,2) AND `posts`.`deleted_at` IS NULL
[{Model:0xc0001b5380 ID:1 PostTitle:example title 1 PostSlug:ex-1 PostCategoryID:1} {Model:0xc0001b53e0 ID:2 PostTitle:example title 2 PostSlug:ex-2 PostCategoryID:2} {Model:0xc0001b5440 ID:3 PostTitle:example title 3 PostSlug:ex-3 PostCategoryID:1}]
My MySQL Example Data:
post_categories
posts