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March 03, 2025 01:00
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Realm Database Cheatsheet

A concise guide to using Realm, covering schema definition, CRUD operations, queries, and relationships.

Core Concepts & Setup

Realm Fundamentals

Realm: A mobile database solution that offers an alternative to SQLite and Core Data. It’s designed for speed and ease of use.

Key Features:

  • Real-time: Data changes are immediately reflected.
  • Cross-platform: Supports multiple platforms (iOS, Android, React Native, etc.).
  • Object-oriented: Data is represented as objects.

Data Model: Realm uses a schema to define the structure of your data. Models are defined as classes.

Installation (Swift): Add realm-swift to your Podfile or use Swift Package Manager.

Importing Realm:

import RealmSwift

Configuration

Default Realm

The default Realm is suitable for most basic use cases. It stores data in the app’s default location.

Custom Realm Configuration

Use Realm.Configuration to customize Realm’s behavior, like specifying a different file path or encryption key.

In-Memory Realm

Useful for testing. Data is not persisted to disk.

Realm.Configuration.defaultConfiguration = Realm.Configuration(inMemoryIdentifier: "MyInMemoryRealm")

Error Handling

Realm throws exceptions for various errors. Wrap Realm operations in do-catch blocks to handle them.

Common Errors:

  • Invalid schema: Incorrect property types or missing primary keys.
  • Migration required: Schema changes necessitate a migration.

Example:

do {
    let realm = try Realm()
} catch {
    print("Error initializing Realm: (error)")
}

Defining Realm Models

Basic Model Definition

Realm models are defined as classes that inherit from Object.

Properties must be declared with the @objc dynamic var prefix to enable Realm’s change tracking.

Example:

class Dog: Object {
    @objc dynamic var name = ""
    @objc dynamic var age = 0
}

Supported Data Types

Int

Integer numbers.

Double, Float

Floating-point numbers.

String

Textual data.

Bool

Boolean values (true/false).

Date

Date and time values.

Data

Binary data.

Optional Properties

Properties can be declared as optional using ?.

Optional properties can store nil values.

Example:

class Person: Object {
    @objc dynamic var name: String? = nil
}

Ignored Properties

Properties marked with @objc ignore are not persisted to the Realm file.

Useful for temporary or calculated values.

Example:

class Rectangle: Object {
    @objc dynamic var width = 0
    @objc dynamic var height = 0
    @objc ignore var area: Int {
        return width * height
    }
}

CRUD Operations

Creating Objects

Create instances of your Realm model classes and add them to the Realm.

Example:

do {
    let realm = try Realm()
    try realm.write {
        let dog = Dog()
        dog.name = "Buddy"
        dog.age = 3
        realm.add(dog)
    }
} catch {
    print("Error creating object: (error)")
}

Reading Objects

Use Realm queries to retrieve objects.

Example:

do {
    let realm = try Realm()
    let dogs = realm.objects(Dog.self)
    for dog in dogs {
        print("Dog name: (dog.name), age: (dog.age)")
    }
} catch {
    print("Error reading objects: (error)")
}

Updating Objects

Update objects within a write transaction.

Example:

do {
    let realm = try Realm()
    let dog = realm.objects(Dog.self).first
    try realm.write {
        dog?.age = 4
    }
} catch {
    print("Error updating object: (error)")
}

Deleting Objects

Delete objects within a write transaction.

Example:

do {
    let realm = try Realm()
    let dog = realm.objects(Dog.self).first
    try realm.write {
        if let dogToDelete = dog {
            realm.delete(dogToDelete)
        }
    }
} catch {
    print("Error deleting object: (error)")
}

Querying Realm Data

Basic Queries

Realm uses a query language similar to NSPredicate.

Use realm.objects(YourModel.self).filter("your_query") to filter results.

Example:

let youngDogs = realm.objects(Dog.self).filter("age < 5")

Common Query Operators

=

Equals.

!=

Not equals.

>

Greater than.

<

Less than.

>=

Greater than or equal to.

<=

Less than or equal to.

BEGINSWITH

String starts with.

ENDSWITH

String ends with.

CONTAINS

String contains.

LIKE

String matches a wildcard pattern.

Compound Predicates

Combine predicates using AND, OR, and NOT.

Example:

let query = "age > 2 AND name BEGINSWITH 'B'"
let results = realm.objects(Dog.self).filter(query)

Sorting Results

Use sorted(byKeyPath:ascending:) to sort results.

Example:

let sortedDogs = realm.objects(Dog.self).sorted(byKeyPath: "age", ascending: true)