how preload a full hierarchy in GO using GORM

Question

I have a hierachy composed by several structures

type Entry struct {
    Id          int
    CreatedAt   time.Time
    UpdatedAt   time.Time
    Fields      []Field
}

type SyncField struct {
    Id                   int
    CreatedAt            time.Time
    UpdatedAt            time.Time
    TechnicalName        string
    JsonName             string
    EntryId              int
    Decorators           []Decorator
}

type Decorator struct {
    Id           int
    CreatedAt    time.Time
    UpdatedAt    time.Time
    Name         string
    Description  string
    SortingOrder int
    Params       string
    SyncFieldId  int
}

My DB creation is definded like this :

db.CreateTable(&Entry{})
db.CreateTable(&SyncField{})
db.Model(&Entry{}).Related(&SyncField{}, "EntryId")

db.CreateTable(&Decorator{})
db.Model(&SyncField{}).Related(&Decorator{}, "DecoratorId")

Everything is clear and works fine to preload just one "sub level" of the hierachy like this :

entry := &Entry{Id: i}
iDB.Preload("SyncFields").First(entry)

or

field := &SyncField{Id: idf}
iDB.Preload("Decorators").First(field)

I'm looking for a way using GORM to preload the whole hierachy invoking the "First()" method on one of the root element... I want to load an "Entry" with all its related "Fields" and I also want to have each "Field" preloaded with all its "Decorators"...

This is not feasable using the several "Preload()" functions :

entry := &Entry{Id: i}
iDB.Preload("Decorators").Preload("SyncFields").First(entry)

I understand "iDB.Preload("child1").Preload("child2").First(root)" works when both child1 and child2 are "leaf" of root.

So my question is : is it possible to do this with gorm and if yes what is the best way to load recursively a complete hierachy?

Thanks. Regards

Answer 1

GORM supports nested preloading.

Please refer to the documentation: https://gorm.io/docs/preload.html#Nested-Preloading

Answer 2

my solution:

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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")
    }
}