Database folder and migrations in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when you run rails db:migrate?

In a Rails project, what is the main effect of running the command rails db:migrate?

A It backs up the current database and creates a new empty database.

B It deletes all data from the database and resets the schema to the initial state.

C It generates new migration files based on changes in the models automatically.

D It applies all pending migration files in the <code>db/migrate</code> folder to update the database schema.

💡 Hint

Think about what migrations are for and what the db/migrate folder contains.

Running rails db:migrate applies all migration files that have not yet been run. These files describe changes to the database schema, such as creating or altering tables.

📝 Syntax

intermediate

Identify the correct migration syntax to add a new column

Which of the following migration snippets correctly adds a string column named username to the users table?

A add_column :username, :users, :string

B add_column :users, :username, :string

C add_column :users, :username, string

D add_column users, username, :string

💡 Hint

Remember the order of arguments for add_column is table name, column name, then type.

The correct syntax is add_column :table_name, :column_name, :type. Option B follows this exactly.

🔧 Debug

advanced

Why does this migration fail to run?

Consider this migration code:

class AddAgeToUsers < ActiveRecord::Migration[6.1]
  def change
    add_column :users, :age, integer
  end
end

Why will this migration cause an error when running rails db:migrate?

Ruby on Rails

class AddAgeToUsers < ActiveRecord::Migration[6.1]
  def change
    add_column :users, :age, integer
  end
end

A The migration version [6.1] is not supported.

B The migration class name is incorrect; it should be plural.

C The type 'integer' is not a symbol; it should be :integer.

D The method 'change' is missing a 'super' call.

💡 Hint

Check the data type argument in add_column.

In Rails migrations, data types must be symbols. Using integer without a colon causes a NameError because Ruby treats it as a variable or method name.

❓ state_output

advanced

What is the schema version after running migrations?

After running rails db:migrate, where does Rails store the current schema version number?

A In the <code>schema_migrations</code> table inside the database.

B In the <code>db/schema.rb</code> file as a Ruby constant.

C In the <code>config/database.yml</code> file under version key.

D In the <code>db/migrate</code> folder name.

💡 Hint

Think about how Rails tracks which migrations have been applied.

Rails uses a special table called schema_migrations in the database to record the version numbers of applied migrations. This helps Rails know which migrations are pending.

🧠 Conceptual

expert

Why should migration files be immutable after deployment?

In a team project, why is it important not to edit existing migration files after they have been run in production?

A Because changing migration files can cause inconsistencies between developers' databases and production, leading to errors.

B Because Rails automatically locks migration files after deployment to prevent changes.

C Because migration files are compiled into the application binary and cannot be changed.

D Because editing migration files will delete all data in the database automatically.

💡 Hint

Consider what happens if different environments have different migration histories.

Once migrations are run in production, changing them can cause mismatches in schema versions across environments. This breaks the migration system and can cause runtime errors or data loss.