Migrations for schema changes in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when you run this migration script?

Consider this migration script using a common migration tool in Express apps. What will be the result after running it?

Express

exports.up = function(knex) {
  return knex.schema.table('users', function(table) {
    table.string('nickname').notNullable().defaultTo('guest');
  });
};

exports.down = function(knex) {
  return knex.schema.table('users', function(table) {
    table.dropColumn('nickname');
  });
};

A Removes the 'nickname' column from 'users' table.

B Adds a new column 'nickname' to 'users' table with default 'guest' and disallows null values.

C Creates a new table called 'nickname' with a string column.

D Renames the 'nickname' column to 'guest' in 'users' table.

💡 Hint

Look at the 'up' function and what it does to the 'users' table.

The 'up' function adds a new string column 'nickname' to the 'users' table. It sets it as not nullable and gives a default value 'guest'. The 'down' function reverses this by dropping the column.

📝 Syntax

intermediate

Identify the syntax error in this migration code

Which option correctly identifies the syntax error in this migration script?

Express

exports.up = function(knex) {
  return knex.schema.createTable('posts', function(table) {
    table.increments('id').primary()
    table.string('title').notNullable()
    table.text('content')
    table.timestamps
  });
};

A Missing parentheses after 'table.timestamps' to call the function.

B The 'text' column type is invalid in this context.

C The 'increments' method cannot be chained with 'primary()'.

D Missing semicolon after 'table.string('title').notNullable()'.

💡 Hint

Check how functions are called in JavaScript.

'table.timestamps' is a function and must be called with parentheses 'table.timestamps()' to add created_at and updated_at columns.

🔧 Debug

advanced

Why does this migration fail when adding a non-nullable column?

This migration tries to add a non-nullable column 'age' to 'users' table but fails. Why?

Express

exports.up = function(knex) {
  return knex.schema.table('users', function(table) {
    table.integer('age').notNullable();
  });
};

A Because the 'table' argument is missing in the callback function.

B Because 'integer' is not a valid column type in knex migrations.

C Because existing rows have no value for 'age', violating the non-null constraint.

D Because the migration script is missing a return statement.

💡 Hint

Think about what happens to existing data when adding a non-nullable column without a default.

Adding a non-nullable column without a default value fails if existing rows do not have a value for that column, causing a constraint violation.

🧠 Conceptual

advanced

What is the main purpose of the 'down' function in migration scripts?

Why do migration scripts include a 'down' function alongside the 'up' function?

A To reverse the changes made by the 'up' function, allowing rollback of schema changes.

B To apply additional schema changes after the 'up' function completes.

C To validate the schema after migration is applied.

D To seed the database with initial data.

💡 Hint

Think about how you undo a migration if something goes wrong.

The 'down' function defines how to undo the changes made by 'up', enabling safe rollback of database schema changes.

❓ state_output

expert

What is the state of the 'products' table after running this migration?

After running this migration, what columns will the 'products' table have?

Express

exports.up = function(knex) {
  return knex.schema.table('products', function(table) {
    table.dropColumn('description');
    table.string('summary').notNullable().defaultTo('No summary');
  });
};

A Only the 'description' column remains; 'summary' is not added due to error.

B Both 'description' and 'summary' columns exist with 'summary' nullable.

C The migration fails because you cannot drop and add columns in the same migration.

D The 'description' column is removed; a new 'summary' column is added with default 'No summary' and not nullable.

💡 Hint

Check what happens when you drop a column and add another in one migration.

The migration removes the 'description' column and adds a new 'summary' column with constraints and default value. This is valid and will succeed.