how preload a full hierarchy in GO using GORM

This is already supported, please refer to the documentation: http://jinzhu.me/gorm/crud.html#nested-preloading


my solution:

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Copyright (c) 2017 Zonkiie
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
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package main

/// http://stackoverflow.com/questions/23030884/is-there-a-way-to-create-an-instance-of-a-struct-from-a-string
/// http://stackoverflow.com/questions/7850140/how-do-you-create-a-new-instance-of-a-struct-from-its-type-at-runtime-in-go
/// http://stackoverflow.com/questions/29435783/gorm-golang-orm-associations

import (
    "github.com/jinzhu/gorm"
    _ "github.com/jinzhu/gorm/dialects/sqlite"
    "fmt"
    "reflect"
    "regexp"
    "encoding/json"
    "encoding/xml"
    //"strconv"
    "os"
    "flag"
    "strings"
)

type rs struct {
    //Parent    *rs `gorm:"ForeignKey:ID;AssociationForeignKey:ParentID"`
    Childs  []*rs   `gorm:"ForeignKey:ID;AssociationForeignKey:ParentID" walkrec:"true"`
    ID  int64
    ParentID    int64   `gorm:"column:ParentID"`
    Value   string
    Sub []rs_sub    `gorm:"ForeignKey:ID;AssociationForeignKey:Rs_ID" walkrec:"true"`
}

type rs_sub struct {
    ID  int64
    Rs_ID   int64   `gorm:"column:rs_id"`
    Value   string
}

var db *gorm.DB

func populateDb(db *gorm.DB) {

    rs1 := rs{ID: 1, ParentID: 0, Value: "root"}
    db.Save(&rs1)
    rs2 := rs{ID: 2, ParentID: 1, Value: "Child1"}
    db.Save(&rs2)
    rs3 := rs{ID: 3, ParentID: 2, Value: "Child2"}
    db.Save(&rs3)
    rs4 := rs{ID: 4, ParentID: 3, Value: "Child3"}
    db.Save(&rs4)
    rs5 := rs{ID: 5, ParentID: 3, Value: "Child4"}
    db.Save(&rs5)
    rs_s := rs_sub{ID:1, Rs_ID:4, Value: "SubChild1"}
    db.Save(&rs_s)
}

func PStdErr(format string, args ...interface{}) {
    fmt.Fprintf(os.Stderr, format, args...)
}

func JsonMarshal(data interface{}) string {
    b, err := json.Marshal(data)
    if err != nil {
        return "Error"
    }
    return string(b[:])
}

func XmlMarshal(data interface{}) string {
    b, err := xml.Marshal(data)
    if err != nil {
        return "Error"
    }
    return string(b[:])
}

func InitDB() *gorm.DB {
db, err := gorm.Open("sqlite3", ":memory:")
if err != nil {
    panic("failed to connect database")
}
db.Exec("DROP TABLE IF EXISTS rs;")

    db.SingularTable(true)

// Migrate the schema
db.AutoMigrate(&rs{})
db.AutoMigrate(&rs_sub{})

populateDb(db)
//db.LogMode(true)
return db

}

// This function fetches all related objects from a given object in the data parameter.
// The struct must be fully tagged, we don't recognize automatically related IDs and so on.
// The function works only with not combined keys.
// Every field which should be fetched must be tagged with:
// walkrec:"true" gorm:"ForeignKey:ID;AssociationForeignKey:ForeignKey"
// See: http://stackoverflow.com/questions/24537525/reflect-value-fieldbyname-causing-panic
// See: http://stackoverflow.com/questions/34493062/how-to-reflect-struct-recursive-in-golang
func fetchRec(db *gorm.DB, data interface{}) {
    // With data *rs: Type: *main.rs
    // With data interface{}: *main.rs
    var ref reflect.Value
    if reflect.TypeOf(data).Kind() == reflect.Struct {
        ref = reflect.ValueOf(data)
    } else if reflect.TypeOf(data).Kind() == reflect.Ptr {
        ref = reflect.Indirect(reflect.ValueOf(data))
    }
    if ref.Type().Kind() == reflect.Slice {
        for i := 0; i < ref.Len(); i++ {
            if ref.Index(i).Type().Kind() == reflect.Ptr {
                fetchRec(db, ref.Index(i).Elem().Addr().Interface())
            } else if ref.Index(i).Type().Kind() == reflect.Struct {
                // What should we do here?
            }
        }

    } else if ref.Type().Kind() == reflect.Struct {
        for i := 0; i < ref.NumField(); i++ {
            var IDFieldRaw string
            var IDFields []string
            var RefFieldRaw string
            var RefFields []string
            var re *regexp.Regexp
            var matches []string

            if ref.Field(i).CanAddr() && strings.EqualFold(ref.Type().Field(i).Tag.Get("walkrec"), "true") {
                gormflags := ref.Type().Field(i).Tag.Get("gorm")
                if gormflags == "" {
                    panic("No gorm flags found!")
                } else {
                    re = regexp.MustCompile(`\bForeignKey:([a-zA-Z0-9_,]+)\b`)
                    matches = re.FindStringSubmatch(gormflags)
                    if len(matches) == 2 {
                        IDFieldRaw = matches[1]
                        IDFields = strings.Split(IDFieldRaw, ",")
                    }
                    re = regexp.MustCompile(`\bAssociationForeignKey:([a-zA-Z0-9_,]+)\b`)
                    matches = re.FindStringSubmatch(gormflags)
                    if len(matches) == 2 {
                        RefFieldRaw = matches[1]
                        RefFields = strings.Split(RefFieldRaw, ",")
                    }
                }
                if len(IDFields) == 0 { continue }
                if len(RefFields) != 0 {
                    WhereMap := make(map[string]interface{})
                    for fk := 0; fk < len(RefFields); fk++ {
                        WhereMap[RefFields[fk]] = fmt.Sprint(ref.FieldByName(IDFields[fk]))
                    }
                    db.Where(WhereMap).Find(ref.Field(i).Addr().Interface())
                    if ref.Field(i).Addr().Interface() != nil {
                        fetchRec(db, ref.Field(i).Addr().Interface())
                    }
                } else {
                    panic("AssociationForeignKey empty!")
                }
            }
        }
    }
}

func getParams() (id int) {
    flag.IntVar(&id, "id", 1, "the id to fetch")
    flag.Parse()
    return
}

func fetch(db *gorm.DB, id interface{}) (d rs, found bool) {

    //db.First(&d, id)
    found = false
    found = !db.Find(&d, id).RecordNotFound()
    if found {
        fetchRec(db, &d)
    }
    return
}

// Execute this program. For example:
// go run main.go --id=2
func main() {
    db = InitDB()
    defer db.Close()
    id := getParams()
    PStdErr("Loading data with ID %d\n", id)
    rs, found := fetch(db, id)
    if found {
        fmt.Print(XmlMarshal(rs) + "\n")
    }
}

Tags:

Go

Go Gorm