How to insert many records in Realm with Swift?

Well after many tests I found a way to reduce the time to only 1-2 seconds of data creation to insert and 7-9 seconds of insertion (it can be lowered more if fewer records are shown, for example if I show when 1000 insertions is reduced to 4-6 seconds).

The first thing I have to say is that the objects were wrong, although in sqlite the many-to-many relationships are made with an auxiliary table, here it is completely superfluous, since one can insert the same record several times in an array, for so the objects remain like this:

   class ProductRealm: Object {
        public dynamic var id: String = ""
        public dynamic var code: String = ""
        public dynamic var detail: String = ""
        public dynamic var onHand: Int = 0
        public dynamic var price: Double = 0
        public dynamic var isSerialized: Int = 0
        public dynamic var isLotNumber: Int = 0
        override static func primaryKey() -> String? {
            return "id"
        }
    }

   class GroupRealm: Object {
        public dynamic var id = 1
        public dynamic var name = ""
        public dynamic var update = Date()
        let products = List<ProductRealm>()
        override static func primaryKey() -> String? {
            return "id"
        }
    }

This means that unlike sqlite it is only necessary to insert the data in a single "table" and its references in the other "table" which makes it much simpler and improves performance a bit, however the insertion times They were still very high.

After many tests it occurred to me to use the jsonObject to insert, since the idea is to bring these records from a server, and to my surprise this greatly reduced the insertion time compared to the dictionaries, so the final code would look like this :

//
//  ViewController.swift
//  TestRealm
//
//  Created by Sergio Castro on 10/02/17.
//  Copyright © 2017 BasTechSoluctions. All rights reserved.
//

import UIKit
import RealmSwift

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        // Mostrar la ubicación del proyecto para ubicar la base de datos
        print(FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!.absoluteString)


        DispatchQueue(label: "background").async { //Abrir un nuevo hilo para insertar de forma asíncrona
            autoreleasepool {

                let date1 = Date() // Inicia el conteo del tiempo

                let realm = try! Realm() // Instanciar realm en el hilo nuevo

                let products = realm.objects(ProductRealm.self) //Crear una lista de todos los productos

                //Crear grupo
                let group = GroupRealm()
                group.id = 1
                group.name = "test"

                try! realm.write { //Insertar/Reemplazar grupo, sin productos
                    realm.create(GroupRealm.self, value: group, update: true)
                }

                ///Crear json
                //Primero creo un String, donde ingreso todos los datos en formato [{},{},...]
                var stringJson = "["
                for i in 0..<30000 {
                    if(i > 0){ stringJson.append(",") }
                    let dataOrigin = "{\"id\": \"\(i)\", \"code\": \"\", \"detail\": \"\", \"onHand\": 0, \"price\": 0, \"isSerialized\": 0, \"isLotNumber\": 0}"
                    stringJson.append("\(dataOrigin)")
                }
                stringJson.append("]")

                //Convertir el String en Datos y luego pasarlo a jsonObject (esto para simular el proceso de descarga de datos)
                let data = stringJson.data(using: .utf8)!
                let jsonP = try! JSONSerialization.jsonObject(with: data, options: .allowFragments) as! [Any]

                //Mostrar el tiempo empleado para la creación del json en consola, el tiempo está en segundos por defecto
                let date2 = Date()
                var time = date2.timeIntervalSince(date1)
                print("Json creation in seconds \(time)")

                //Recorrer el json en grupos de 100, tantas veces haga falta para recorrer por completo el json, en este caso 300 veces
                let limit = 100
                for c in 0..<(jsonP.count / limit){
                    //Insertar/Reemplazar los grupos de datos
                    realm.beginWrite()
                    for i in 0..<limit{
                        let row = jsonP[i + (c*limit)]
                        realm.create(ProductRealm.self, value: row, update: true)
                    }
                    try! realm.commitWrite()
                    //Mostrar en consola la cantidad de procesos realizados vs la cantidad de registros actuales
                    print("\((c+1)*limit) / \(products.count)")
                }

                group.products.append(objectsIn: products) //Agregar los productos al grupo, aquí se pueden poner filtros

                //Actualizar el grupo, para que el array products tenga una referencia a sus productos correspondientes
                try! realm.write {
                    realm.create(GroupRealm.self, value: group, update: true)
                }

                // Calcular el tiempo de inserción y mostrarlo en consola
                let date3 = Date()
                time = date3.timeIntervalSince(date2)
                print("Insert time in seconds \(time)")

                // Calcular el tiempo total del calculo
                time = date3.timeIntervalSince(date1)
                print("total time in seconds \(time)")

            }
        }
    }
}


class ProductRealm: Object {
    public dynamic var id: String = ""
    public dynamic var code: String = ""
    public dynamic var detail: String = ""
    public dynamic var onHand: Int = 0
    public dynamic var price: Double = 0
    public dynamic var isSerialized: Int = 0
    public dynamic var isLotNumber: Int = 0
    override static func primaryKey() -> String? {
        return "id"
    }
}

class GroupRealm: Object {
    public dynamic var id = 1
    public dynamic var name = ""
    public dynamic var update = Date()
    let products = List<ProductRealm>()
    override static func primaryKey() -> String? {
        return "id"
    }
}

Well I'll leave it that way, but if you find a way to optimize it I'd really appreciate it.

What is mainly making it take longer to insert it is opening and closing several write transactionswithin the for.

This would be an inefficient or incorrect way:

let realm = try Realm()

for i in 0..<300 {

    try realm.write {

        // Inserts
    }

}

Instead, this other would be the most efficient and correct:

let realm = try Realm()

try realm.write {

    for i in 0..<300 {

        // Inserts

    }

}

For your particular case, I would do the following:

let realm = try! Realm()

let group = GroupRealm()

group.id = 1
group.name = "test"

realm.beginWrite() // -------------- Inicio de escritura

realm.add(group)

var product = [String:AnyObject]()
product["id"] = "" as AnyObject!
product["code"] = ("") as AnyObject!
product["detail"] = ("") as AnyObject!
product["onHand"] = (0) as AnyObject!
product["price"] = (0) as AnyObject!
product["isSerialized"] = (0) as AnyObject!
product["isLotNumber"] = (0) as AnyObject!

var groupProduct = [String:AnyObject]()
groupProduct["group"] = group as AnyObject!

for r in 0..<300 {

    for i in 0..<100 {
        product["id"] = "\(i)-\(r)" as AnyObject!
        realm.create(ProductRealm.self, value: product, update: true)

        groupProduct["id"] = "\(i)-\(r)" as AnyObject!
        groupProduct["product"] = product as AnyObject!
        realm.create(GroupProductRealm.self, value: groupProduct, update: true)
    }

}

try! realm.commitWrite() // -------------- Fin de escritura