Heroku deployment in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when you run heroku run rails db:migrate?

After deploying a Rails app to Heroku, you run heroku run rails db:migrate. What does this command do?

A It runs the database migrations on the Heroku production database.

B It runs migrations locally on your computer's database.

C It restarts the Heroku app without running migrations.

D It deletes the Heroku database and creates a new one.

💡 Hint

Think about where the app is running and where the database lives on Heroku.

The command heroku run rails db:migrate runs migrations on the Heroku app's production database, updating its schema.

📝 Syntax

intermediate

Which Procfile entry correctly starts a Rails web server on Heroku?

You want to tell Heroku how to start your Rails app web server. Which Procfile line is correct?

A web: rails server -p 3000

B web: bundle exec rails server -p $PORT

C web: rails start

D web: bundle exec puma

💡 Hint

Heroku assigns a port dynamically. The command must use that port.

The correct Procfile entry uses $PORT so Heroku can assign the port dynamically. It also uses bundle exec rails server to start the server.

🔧 Debug

advanced

Why does this Heroku deployment fail with a PG::ConnectionBad error?

You deployed your Rails app to Heroku but get this error in logs: PG::ConnectionBad: could not connect to server. What is the most likely cause?

A The <code>DATABASE_URL</code> environment variable is missing or incorrect.

B The Rails app is missing the <code>sqlite3</code> gem in production.

C The <code>Procfile</code> is missing the web server command.

D The <code>Gemfile.lock</code> is not committed to git.

💡 Hint

Heroku uses DATABASE_URL to connect to the database.

The error means the app cannot connect to the Postgres database. Usually, this happens if DATABASE_URL is missing or wrong.

❓ state_output

advanced

What is the effect of running heroku config:set RAILS_ENV=production?

You run heroku config:set RAILS_ENV=production on your app. What does this change do?

A It restarts the app without changing any environment variables.

B It disables asset precompilation during deployment.

C It sets the environment variable only for your local machine.

D It sets the Rails environment to production on Heroku, affecting config and logging.

💡 Hint

Environment variables on Heroku control app behavior in the cloud.

Setting RAILS_ENV=production on Heroku tells Rails to run in production mode, which changes logging, caching, and other settings.

🧠 Conceptual

expert

Why must you precompile assets before deploying a Rails app to Heroku?

Rails uses asset precompilation before deployment. Why is this step necessary on Heroku?

A To create database indexes for faster queries on Heroku Postgres.

B To convert Ruby code into JavaScript for Heroku's server environment.

C To generate optimized CSS and JavaScript files so the app loads faster in browsers.

D To install all gems needed by the app before deployment.

💡 Hint

Think about what assets are and how browsers use them.

Precompiling assets creates optimized CSS and JS files that browsers can load quickly, improving app performance on Heroku.

Practice

1. What is the main purpose of deploying a Rails app to Heroku?

easy

A. To put the app online without managing servers

B. To write Ruby code faster

C. To create a local database

D. To debug Rails code automatically

Solution

1. Step 1: Understand Heroku's role

   Heroku is a platform that hosts apps so they can be accessed online without you managing the servers.

2. Step 2: Compare options

   Options B, C, and D describe development or debugging tasks, not deployment.

3. Final Answer:

   To put the app online without managing servers -> Option A

4. Quick Check:

   Heroku deployment = online app hosting [OK]

Hint: Heroku hosts apps online, no server setup needed [OK]

Common Mistakes:

• Confusing deployment with coding or debugging
• Thinking Heroku manages local databases
• Assuming Heroku speeds up writing code

2. Which command is used to push your Rails app code to Heroku?

easy

A. heroku push

B. heroku deploy app

C. rails deploy

D. git push heroku main

Solution

1. Step 1: Identify the correct git command

   Heroku uses git to receive code. The correct command is 'git push heroku main' to push the main branch.

2. Step 2: Check other options

   Options A, C, and D are not valid commands for pushing code to Heroku.

3. Final Answer:

   git push heroku main -> Option D

4. Quick Check:

   Push code with git push heroku main [OK]

Hint: Use git push heroku main to deploy code [OK]

Common Mistakes:

• Using 'heroku push' which is invalid
• Trying 'rails deploy' which doesn't exist
• Confusing Heroku CLI commands

3. After pushing your Rails app to Heroku, which command runs database migrations on Heroku?

medium

A. heroku run rails db:migrate

B. rails db:migrate

C. git migrate heroku

D. heroku migrate db

Solution

1. Step 1: Understand migration on Heroku

   To run migrations on Heroku's remote server, you use 'heroku run rails db:migrate'.

2. Step 2: Check other commands

   'rails db:migrate' runs locally, 'git migrate heroku' and 'heroku migrate db' are invalid commands.

3. Final Answer:

   heroku run rails db:migrate -> Option A

4. Quick Check:

   Run migrations remotely with heroku run rails db:migrate [OK]

Hint: Run migrations remotely with heroku run rails db:migrate [OK]

Common Mistakes:

• Running migrations locally instead of on Heroku
• Using invalid git commands for migration
• Confusing command order

4. You pushed your Rails app to Heroku but see an error when opening the app URL. What is a common cause?

medium

A. You did not install Ruby locally

B. You used 'git push origin main' instead of 'git push heroku main'

C. You forgot to run database migrations on Heroku

D. You opened the wrong browser

Solution

1. Step 1: Identify common deployment errors

   Forgetting to run migrations on Heroku often causes errors because the database schema is not updated.

2. Step 2: Evaluate other options

   You used 'git push origin main' instead of 'git push heroku main' means code wasn't pushed to Heroku, but question says code was pushed. Options A and D are unrelated to deployment errors.

3. Final Answer:

   You forgot to run database migrations on Heroku -> Option C

4. Quick Check:

   Missing migrations cause app errors on Heroku [OK]

Hint: Run migrations after push to avoid errors [OK]

Common Mistakes:

• Ignoring migrations after deployment
• Confusing git push origin with heroku
• Blaming local environment for Heroku errors

5. You want to deploy a Rails app with environment variables for API keys on Heroku. Which is the correct way to set them before deployment?

hard

A. Add keys to config/application.rb file

B. Use 'heroku config:set KEY=value' command

C. Store keys in a local .env file only

D. Push keys directly in your code files

Solution

1. Step 1: Understand environment variables on Heroku

   Heroku uses 'heroku config:set' to securely store environment variables like API keys.

2. Step 2: Evaluate other options

   Adding keys to config/application.rb or pushing keys in code risks exposure. Local .env files are not sent to Heroku.

3. Final Answer:

   Use 'heroku config:set KEY=value' command -> Option B

4. Quick Check:

   Set env vars securely with heroku config:set [OK]

Hint: Use heroku config:set to add env variables securely [OK]

Common Mistakes:

• Hardcoding keys in code risking leaks
• Relying on local .env files for Heroku
• Editing config/application.rb for secrets