Gorm Query Customized Join Extra Column

I am trying to get extra columns from a many2many relationships on Gorm. Example

1. Part

type Part struct {
    Id unit
    Name string
 }

2. Diagram

type Diagram struct {
    Id unit
    Name string
    Parts []Part `gorm:"many2many:diagram_parts;"`
 }

3. 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?

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)
}