Realm database in React Native

Introduction

Realm database helps your app save and read data quickly and easily on the device. It works offline and keeps your data safe.

You want to save user notes or settings locally on the phone.

Your app needs to work without internet and sync later.

You want to store lists of items like tasks or contacts.

You want a simple way to manage data without complex setup.

Syntax

React Native

import Realm from 'realm';

const CarSchema = {
  name: 'Car',
  properties: {
    make: 'string',
    model: 'string',
    miles: 'int',
  },
};

const realm = new Realm({schema: [CarSchema]});

Define your data structure using a schema object.

Create a Realm instance with your schema to start using the database.

Examples

This schema defines a Dog with a name and age.

React Native

const DogSchema = {
  name: 'Dog',
  properties: {
    name: 'string',
    age: 'int',
  },
};

Add a new Car object inside a write transaction.

React Native

realm.write(() => {
  realm.create('Car', {make: 'Toyota', model: 'Corolla', miles: 10000});
});

Get all Car objects stored in the database.

React Native

const cars = realm.objects('Car');

Sample App

This React Native app uses Realm to store and show a list of cars. It adds a car if none exists and displays all cars stored.

React Native

import React, {useEffect, useState} from 'react';
import {View, Text} from 'react-native';
import Realm from 'realm';

const CarSchema = {
  name: 'Car',
  properties: {
    make: 'string',
    model: 'string',
    miles: 'int',
  },
};

export default function App() {
  const [cars, setCars] = useState([]);

  useEffect(() => {
    const realm = new Realm({schema: [CarSchema]});

    // Add a car if none exists
    realm.write(() => {
      if (realm.objects('Car').length === 0) {
        realm.create('Car', {make: 'Honda', model: 'Civic', miles: 5000});
      }
    });

    // Get all cars
    const allCars = realm.objects('Car');
    setCars(allCars.map(car => `${car.make} ${car.model} - ${car.miles} miles`));

    // Cleanup
    return () => {
      realm.close();
    };
  }, []);

  return (
    <View style={{flex: 1, justifyContent: 'center', alignItems: 'center'}}>
      <Text style={{fontSize: 20, marginBottom: 10}}>Cars in Realm DB:</Text>
      {cars.map((car, index) => (
        <Text key={index}>{car}</Text>
      ))}
    </View>
  );
}

Important Notes

Always open Realm inside useEffect or lifecycle methods to avoid multiple instances.

Use realm.write() to add or change data safely.

Remember to close Realm when the component unmounts to free resources.

Summary

Realm stores data locally on the device for fast access.

Define schemas to tell Realm what data looks like.

Use write transactions to add or update data.

Practice

1. What is the primary purpose of using Realm database in a React Native app?

easy

A. To handle app navigation between screens

B. To store data locally on the device for fast access

C. To style the app's user interface

D. To connect the app to a remote SQL server

Solution

1. Step 1: Understand Realm's role

   Realm is a local database designed to store data on the device itself.

2. Step 2: Compare options

   Options A, B, and C describe unrelated functions like navigation, remote servers, and UI styling, which Realm does not handle.

3. Final Answer:

   To store data locally on the device for fast access -> Option B

4. Quick Check:

   Realm = local data storage [OK]

Hint: Realm is for local data, not UI or navigation [OK]

Common Mistakes:

• Confusing Realm with remote databases
• Thinking Realm manages UI or navigation
• Assuming Realm is a cloud service

2. Which of the following is the correct way to define a Realm schema for a Task object with a string name and a boolean completed property?

easy

A. const TaskSchema = {name: 'Task', properties: {name: 'string', completed: 'bool'}};

B. const TaskSchema = {name: 'Task', properties: {name: 'string', completed: 'boolean'}};

C. const TaskSchema = {name: 'Task', properties: {name: 'String', completed: 'Bool'}};

D. const TaskSchema = {name: 'Task', properties: {name: String, completed: Boolean}};

Solution

1. Step 1: Check Realm property types

   Realm expects property types as lowercase strings like 'string' and 'bool'.

2. Step 2: Compare options

   const TaskSchema = {name: 'Task', properties: {name: 'string', completed: 'bool'}}; uses correct lowercase strings. Options A, C, and D use incorrect types or JavaScript types instead of strings.

3. Final Answer:

   const TaskSchema = {name: 'Task', properties: {name: 'string', completed: 'bool'}}; -> Option A

4. Quick Check:

   Property types are lowercase strings [OK]

Hint: Use lowercase strings for property types in schema [OK]

Common Mistakes:

• Using JavaScript types instead of strings
• Capitalizing type names incorrectly
• Omitting the 'properties' key

3. Given the following code snippet, what will be the output of console.log(tasks.length);?

const realm = await Realm.open({schema: [TaskSchema]});
realm.write(() => {
  realm.create('Task', {name: 'Buy milk', completed: false});
  realm.create('Task', {name: 'Walk dog', completed: true});
});
const tasks = realm.objects('Task');
console.log(tasks.length);

medium

A. 0

B. 1

C. 2

D. Undefined

Solution

1. Step 1: Analyze write transaction

   Two Task objects are created inside the write block, so two entries are added.

2. Step 2: Check objects query

   realm.objects('Task') returns all Task objects, so length should be 2.

3. Final Answer:

   2 -> Option C

4. Quick Check:

   Created 2 tasks, length = 2 [OK]

Hint: Count created objects inside write block [OK]

Common Mistakes:

• Forgetting to use write block for create
• Assuming objects() returns empty before commit
• Confusing length with undefined

4. What is wrong with this Realm write transaction code? (Assume the TaskSchema defines name as the primary key)

realm.write(() => {
  realm.create('Task', {name: 'Read book', completed: false});
});
realm.write(() => {
  realm.create('Task', {name: 'Read book', completed: true});
});

medium

A. The second create will throw an error because a Task with the same primary key already exists

B. You cannot call realm.write() more than once

C. The second create will update the existing object automatically

D. There is no error; this code works fine

Solution

1. Step 1: Check schema requirements

   With name as primary key, creating an object with the same primary key without update flag causes error.

2. Step 2: Analyze code behavior

   The first create succeeds. The second create uses the same name (primary key) without update flag, so it throws an error.

3. Final Answer:

   The second create will throw an error because a Task with the same primary key already exists -> Option A

4. Quick Check:

   Duplicate primary key without update flag causes error [OK]

Hint: Use primary key and update flag to avoid create errors [OK]

Common Mistakes:

• Assuming multiple writes are forbidden
• Thinking create auto-updates without flags
• Ignoring primary key requirements

5. You want to update the completed status of a Task object named 'Laundry' to true. (Assume the Task schema defines name as the primary key) Which code snippet correctly performs this update in Realm?

hard

A. realm.write(() => { realm.objects('Task').filtered('name == "Laundry"').completed = true; });

B. realm.write(() => { realm.create('Task', {name: 'Laundry', completed: true}); });

C. realm.write(() => { realm.update('Task', {name: 'Laundry', completed: true}); });

D. realm.write(() => { realm.create('Task', {name: 'Laundry', completed: true}, true); });

Solution

1. Step 1: Understand update with create

   Realm updates existing objects if you pass true as the third argument to create.

2. Step 2: Check each option

   realm.write(() => { realm.create('Task', {name: 'Laundry', completed: true}, true); }); uses true flag correctly. realm.write(() => { realm.create('Task', {name: 'Laundry', completed: true}); }); tries to create without update flag, causing error. realm.write(() => { realm.objects('Task').filtered('name == "Laundry"').completed = true; }); tries to assign directly to filtered results, which is invalid. realm.write(() => { realm.update('Task', {name: 'Laundry', completed: true}); }); uses a non-existent update method.

3. Final Answer:

   realm.write(() => { realm.create('Task', {name: 'Laundry', completed: true}, true); }); -> Option D

4. Quick Check:

   Use create with true flag to update [OK]

Hint: Use true flag in create to update existing objects [OK]

Common Mistakes:

• Forgetting the update flag for updates
• Trying to assign directly to query results
• Using non-existent Realm methods