Realm database in React Native - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What is the output of this Realm query code?

Given a Realm database with a schema for Task objects having a completed boolean field, what will be the length of completedTasks after running this code?

React Native

const realm = await Realm.open({schema: [{name: 'Task', properties: {name: 'string', completed: 'bool'}}]});
realm.write(() => {
  realm.create('Task', {name: 'Buy milk', completed: true});
  realm.create('Task', {name: 'Walk dog', completed: false});
  realm.create('Task', {name: 'Read book', completed: true});
});
const completedTasks = realm.objects('Task').filtered('completed == true');
console.log(completedTasks.length);

A 3

B 0

C 1

D 2

💡 Hint

Count how many tasks have completed set to true.

Only two tasks have completed set to true: 'Buy milk' and 'Read book'. The filtered query returns those two.

❓ lifecycle

intermediate

What happens if you forget to close a Realm instance in React Native?

In React Native, if you open a Realm instance but do not call realm.close() when the component unmounts, what is the likely outcome?

React Native

import React, {useEffect} from 'react';
import Realm from 'realm';

export default function MyComponent() {
  useEffect(() => {
    Realm.open({schema: [{name: 'Note', properties: {text: 'string'}}]}).then(realm => {
      // Use realm
    });
    // No realm.close() called
  }, []);
  return null;
}

A Memory leak and possible app crash due to open Realm instances.

B Realm data will be lost immediately.

C App will throw a syntax error at runtime.

D Realm automatically closes when component unmounts, no issues.

💡 Hint

Think about resource management and cleanup in apps.

Not closing Realm instances keeps resources open, causing memory leaks and potential crashes.

📝 Syntax

advanced

Which option correctly defines a Realm schema with a primary key?

You want to define a Realm schema for a User model with id as primary key and name as string. Which option is correct?

A const UserSchema = {name: 'User', primaryKey: 'id', properties: {id: 'int', name: 'string'}};

B const UserSchema = {name: 'User', properties: {id: 'int', name: 'string', primaryKey: 'id'}};

C const UserSchema = {name: 'User', properties: {id: 'int', name: 'string'}, primaryKey: 'id'};

D const UserSchema = {name: 'User', properties: {id: 'int', name: 'string'}, primary_key: 'id'};

💡 Hint

Check the correct placement and spelling of primaryKey in schema object.

The primaryKey property must be at the root level of the schema object, not inside properties, and spelled exactly 'primaryKey'.

❓ navigation

advanced

What is the effect of this Realm write transaction in React Native?

Consider this code snippet inside a React Native app using Realm. What will be the value of task.completed after this write?

React Native

const realm = await Realm.open({schema: [{name: 'Task', properties: {name: 'string', completed: 'bool'}}]});
realm.write(() => {
  let task = realm.create('Task', {name: 'Clean house', completed: false});
  task.completed = true;
});
const task = realm.objects('Task').filtered('name == "Clean house"')[0];
console.log(task.completed);

A false

B true

C undefined

D Throws an error

💡 Hint

Think about how Realm transactions update objects.

Inside a write transaction, updating the property sets it in the database. The final value is true.

🔧 Debug

expert

Why does this Realm query throw an error?

This code throws an error when running the filtered query. What is the cause?

React Native

const realm = await Realm.open({schema: [{name: 'Person', properties: {name: 'string', age: 'int'}}]});
realm.write(() => {
  realm.create('Person', {name: 'Alice', age: 30});
});
const results = realm.objects('Person').filtered('age > "25"');

A Filtering with a string "25" instead of a number causes a type error.

B The schema is missing a primary key, causing query failure.

C The write transaction is missing a commit call.

D Realm does not support comparison operators in filtered queries.

💡 Hint

Check the data types used in the filter expression.

The filter compares an integer field to a string literal, causing a type mismatch error.