Working with Realm in iOS

26 Sep 2017

Introduction

From Realm home page:

Realm Mobile Database is an alternative to SQLite and Core Data. Thanks to its zero-copy design, Realm Mobile Database is much faster than an ORM, and often faster than raw SQLite

From my own view point:

Realm is a modern cross-platform mobile database which is easy to integrate and concise to write codes.

Installation

The simplest way to integrate Swift is via CocoaPod.

pod 'RealmSwift'

If you look for others and full installation guide, please visit here.

Usage

I. Realm Object

Basic
Defining a Realm object is quite straight-forward as a normal Swift class. Just pay attention that it need to extend Object class and use dynamic keyword for its properties.

import RealmSwift
class Student: Object{
  dynamic var firstName: String = ""
  dynamic var lastName: String = ""
}

Primary Key
In case of using primary key, it requires to override primaryKey() function as below.

import RealmSwift
class Student: Object{
  dynamic var id: Int = -1
  dynamic var firstName: String = ""
  dynamic var lastName: String = ""
  override class func primaryKey() -> String? {
    return "id"
  }
}

II. Operation

1. Create

Basic

// Get the default Realm
let realm = try! Realm()
// Add new item
try! realm.write {
  realm.add(student)
}

Auto-increment
In SQL, we can define an ‘auto-increment’ field which will be increased by one automatically every time a new row is added. This property, however, is not introduced in Realm. In case we need an auto-increment id in Realm object, we have to handle it by ourselves manually, but it is trivial.

// Generate auto-increment id manually
private func incrementID() -> Int {
  return (uiRealm.objects(Student.self).max(ofProperty: "id") as Int? ?? 0) + 1
}
// When adding new item
try! realm.write {
  student.id = incrementID()
  realm.add(student)
}

2. Query

As always, querying data is the most frequent operation to a database. Compared with Core Data, Realm provides really concise and simple APIs.

Find all objects
Fetch all objects with just one line of codes.

let allStudents:[Student] = Array(realm.objects(Student.self))

Use primary key
Find an object by its primary key.

let student = realm.object(ofType: Student.self, forPrimaryKey: id)

Use NSPredicate
NSPredicate is a Foundation class that specifies how data should be fetched or filtered. It is used extensively in Apple framework system to filter in-memory data (stored in collections), or to fetch data from Core Data, SQLite.
So, Realm supports querying data by NSPredicate too which makes iOS developer adapt easily. A simple query using NSPredicate looks like the following.

let predicate = NSPredicate(format: "firstName BEGINSWITH [c]%@", inputText)
let students = try Realm().objects(Student.self).filter(predicate).sorted("firstName", ascending: true)

You might need this useful NSPredicate Cheatsheet when building data queries.

3. Update

As other write operations in Realm, update should be performed in realm.write block, otherwise, an exception will be thrown.

// Retrieve the object which will be updated.
// e.g, the 1st student information needs to be updated.
student = realm.objects(Student.self).first

// Update
try! realm.write {
  student.first = "new first name"
  student.last = "new last name"
}

4. Delete

Delete an object

try! realm.write {
  realm.delete(student)
}

Delete all objects

try! realm.write {
  realm.deleteAll()
}

A full and comprehensive documentation of Realm Swift can be found at Realm homepage.

QUANG NGUYEN