Gorm Query Customized Join Extra Column - go

Question

I am trying to get extra columns from a many2many relationships on Gorm. Example
Part
type Part struct {
Id unit
Name string
}
Diagram
type Diagram struct {
Id unit
Name string
Parts []Part `gorm:"many2many:diagram_parts;"`
}
DiagramPart
type DiagramPart struct{
DiagramId uint `gorm:"primaryKey"`
PartId uint `gorm:"primaryKey"`
PartDiagramNumber int
PartNumber string
PartDescription string
}
This is what I have done trying to retrieve PartNumber and PartDescription in Parts.
diagram := &Diagram{}
db := s.db.Where("id = ?", 1).
Preload("Parts", func(db *gorm.DB) *gorm.DB {
return db.Select("parts.*, diagram_parts.part_number, diagram_parts.part_description").
Joins("left join diagram_parts on diagram_parts.part_id = parts.id")
}).
First(diagram)
Unfortunately, I am not able to retrieve part_number, part_description. How should I go about it?

Answer 1

You can add field PartNumber and PartDescription on struct Part OR Diagram, then add tag gorm:"-:migration;->" on than fields to ignore migration and to readonly mode. But on your situation, you can add it in struct Part because you already preload it.
source: https://gorm.io/docs/models.html#Field-Level-Permission
here's the example:
package main
import (
"fmt"
"gorm.io/driver/sqlite"
"gorm.io/gorm"
)
type Part struct {
Id uint `gorm:"primaryKey"`
Name string
PartNumber string `gorm:"-:migration;->"`
PartDescription string `gorm:"-:migration;->"`
}
type Diagram struct {
Id uint `gorm:"primaryKey"`
Name string
Parts []Part `gorm:"many2many:diagram_parts;"`
// PartNumber string `gorm:"-:migration;->"`
// PartDescription string `gorm:"-:migration;->"`
}
type DiagramPart struct {
DiagramId uint `gorm:"primaryKey"`
PartId uint `gorm:"primaryKey"`
PartDiagramNumber int
PartNumber string
PartDescription string
}
func main() {
db, err := gorm.Open(sqlite.Open("test.db"), &gorm.Config{})
if err != nil {
panic("failed to connect database")
}
db.AutoMigrate(&Diagram{}, &Part{}, &DiagramPart{})
diagram := &Diagram{}
err = db.Debug().Where("id = ?", 1).
// Select("diagrams.*, diagram_parts.part_number, diagram_parts.part_description").
Preload("Parts", func(db *gorm.DB) *gorm.DB {
return db.Select("parts.*, diagram_parts.part_number, diagram_parts.part_description").
Joins("left join diagram_parts on diagram_parts.part_id = parts.id")
}).
// Joins("left join diagram_parts on diagram_parts.diagram_id = diagrams.id").
First(diagram).Error
if err != nil {
panic(err)
}
fmt.Printf("diagram: %v\n", diagram)
fmt.Println("part number:", diagram.Parts[0].PartNumber)
}