Running and rolling back migrations in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens after running rails db:migrate?

You run rails db:migrate in your Rails project. What is the immediate effect on your database?

A All migrations are deleted from the migrations folder to prevent rerunning.

B The database is reset to its initial empty state, removing all tables.

C The Rails server automatically restarts to apply migration changes.

D The database schema is updated to include all pending migrations, and a record is added to the schema_migrations table.

💡 Hint

Think about what migrations do to the database structure and how Rails tracks them.

Running rails db:migrate applies all pending migrations to update the database schema. Rails records which migrations have run in the schema_migrations table to avoid rerunning them.

❓ state_output

intermediate

What is the state of the database after rails db:rollback?

After running rails db:migrate to apply three migrations, you run rails db:rollback. What is the state of the database schema?

A The last applied migration is reversed, so the database schema reflects only the first two migrations.

B All migrations are reversed, and the database schema is empty.

C The database schema remains unchanged because rollback only affects migration files.

D The first migration is reversed, but the last two remain applied.

💡 Hint

Rollback undoes migrations one step at a time, starting from the most recent.

rails db:rollback reverses the last migration that was applied, updating the database schema accordingly. It does not undo all migrations at once.

📝 Syntax

advanced

Which command rolls back the last two migrations?

You want to undo the last two migrations you applied. Which command correctly does this?

A rails db:migrate:down VERSION=2

B rails db:rollback --count=2

C rails db:rollback STEP=2

D rails db:reset STEP=2

💡 Hint

Look for the option that specifies how many steps to rollback.

The STEP option tells Rails how many migrations to rollback. rails db:rollback STEP=2 rolls back the last two migrations.

🔧 Debug

advanced

Why does rails db:rollback fail with 'No migrations to rollback'?

You run rails db:rollback but get the error 'No migrations to rollback'. What is the most likely reason?

A There are no migrations applied yet, so nothing to rollback.

B The migration files are missing from the project folder.

C The database connection is not established.

D The Rails version does not support rollback commands.

💡 Hint

Rollback only works if migrations have been applied before.

If no migrations have been applied, Rails cannot rollback anything and shows this error.

🧠 Conceptual

expert

What is the effect of rails db:migrate:redo?

You run rails db:migrate:redo. What does this command do to your database schema?

A It deletes the last migration file and updates the schema accordingly.

B It rolls back the last migration and then reapplies it, effectively refreshing the last migration.

C It applies all pending migrations without rolling back any.

D It resets the entire database and reapplies all migrations from scratch.

💡 Hint

Think about what 'redo' means in terms of undoing and reapplying.

rails db:migrate:redo rolls back the last migration and then runs it again. This is useful for testing or fixing the last migration.

Practice

1. What does running a migration in Rails do?

easy

A. Applies changes to the database schema step-by-step

B. Deletes the entire database

C. Creates a backup of the database

D. Restarts the Rails server

Solution

1. Step 1: Understand migration purpose

   Migrations are designed to change the database schema in a controlled way.

2. Step 2: Identify what running migration does

   Running a migration applies these schema changes step-by-step to the database.

3. Final Answer:

   Applies changes to the database schema step-by-step -> Option A

4. Quick Check:

   Running migration = apply schema changes [OK]

Hint: Running migration updates database schema stepwise [OK]

Common Mistakes:

• Thinking it deletes the database
• Confusing migration with backup
• Assuming it restarts the server

2. Which command correctly rolls back the last migration in Rails?

easy

A. rails db:reset

B. rails db:rollback

C. rails db:migrate:down

D. rails db:rollback STEP=2

Solution

1. Step 1: Recall rollback command syntax

   The basic command to undo the last migration is rails db:rollback.

2. Step 2: Analyze other options

   rails db:migrate:down requires specifying a version, rails db:reset resets the whole database, and rails db:rollback STEP=2 rolls back two steps, not one.

3. Final Answer:

   rails db:rollback -> Option B

4. Quick Check:

   Rollback last migration = rails db:rollback [OK]

Hint: Rollback last migration uses rails db:rollback [OK]

Common Mistakes:

• Using db:reset which resets entire DB
• Using rollback with STEP=2 to undo one migration
• Confusing migrate:down without version

3. Given these commands run in order:
rails db:migrate
rails db:migrate
rails db:rollback STEP=2
What is the state of the database migrations?

medium

A. Only the first migration is applied

B. All migrations are applied

C. No migrations are applied

D. Only the last migration is applied

Solution

1. Step 1: Understand the commands run

   Two migrations are applied by running rails db:migrate twice, then rollback with STEP=2 undoes both.

2. Step 2: Determine final migration state

   Rolling back two steps removes both migrations, so no migrations remain applied.

3. Final Answer:

   No migrations are applied -> Option C

4. Quick Check:

   Rollback STEP=2 undoes two migrations [OK]

Hint: Rollback STEP=2 undoes last two migrations [OK]

Common Mistakes:

• Assuming rollback STEP=2 only undoes one migration
• Thinking migrations remain applied after rollback
• Confusing migrate command effects

4. You run rails db:rollback STEP=3 but get an error saying there are not enough migrations to rollback. What is the likely cause?

medium

A. The STEP option is not supported

B. Rollback only works with STEP=1

C. You need to restart the Rails server first

D. There are fewer than 3 migrations applied

Solution

1. Step 1: Understand rollback STEP behavior

   Rollback with STEP=n tries to undo n migrations if they exist.

2. Step 2: Analyze error cause

   If fewer than 3 migrations are applied, rollback STEP=3 fails because it can't undo more than exist.

3. Final Answer:

   There are fewer than 3 migrations applied -> Option D

4. Quick Check:

   Rollback STEP=n needs at least n migrations applied [OK]

Hint: Rollback STEP=n requires n migrations applied [OK]

Common Mistakes:

• Thinking STEP option is unsupported
• Believing server restart fixes rollback errors
• Assuming rollback only works with STEP=1

5. You have applied 5 migrations. You want to undo the last 3 migrations and then reapply them one by one to test changes. Which sequence of commands achieves this?

hard

A. rails db:rollback STEP=3, then rails db:migrate three times

B. rails db:rollback STEP=3, then rails db:migrate once

C. rails db:migrate:reset, then rails db:migrate

D. rails db:rollback, then rails db:migrate STEP=3

Solution

1. Step 1: Undo last 3 migrations

   Use rails db:rollback STEP=3 to undo the last three migrations at once.

2. Step 2: Reapply migrations one by one

   Run rails db:migrate three times to apply each migration individually.

3. Final Answer:

   rails db:rollback STEP=3, then rails db:migrate three times -> Option A

4. Quick Check:

   Rollback STEP=3 + migrate x3 reapplies migrations [OK]

Hint: Rollback STEP=3 then migrate repeatedly reapplies changes [OK]

Common Mistakes:

• Using migrate once after rollback STEP=3
• Resetting database instead of rolling back
• Trying migrate with STEP option (not valid)