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.

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