Adding and removing columns in Ruby on Rails - Step-by-Step Execution

Concept Flow - Adding and removing columns

Generate Migration

↓

Edit Migration File

↓

Run Migration

↓

Database Schema Updated

↓

Use Updated Model

The flow shows creating a migration, editing it to add or remove columns, running it to update the database, and then using the updated model.

Execution Sample

Ruby on Rails

class AddAgeToUsers < ActiveRecord::Migration[7.0]
  def change
    add_column :users, :age, :integer
  end
end

This migration adds an integer column 'age' to the 'users' table.

Execution Table

Step	Action	Migration File Content	Database Schema Change	Result
1	Generate migration	class AddAgeToUsers < ActiveRecord::Migration[7.0] def change end end	No change yet	Migration file created
2	Edit migration	class AddAgeToUsers < ActiveRecord::Migration[7.0] def change add_column :users, :age, :integer end end	No change yet	Migration file updated with add_column
3	Run migration	Same as step 2	users table gains 'age' column of type integer	Schema updated, column added
4	Use model	N/A	Schema includes 'age' column	Can now read/write 'age' attribute on User
5	Generate remove migration	class RemoveAgeFromUsers < ActiveRecord::Migration[7.0] def change remove_column :users, :age end end	No change yet	Migration file created to remove column
6	Run remove migration	Same as step 5	users table loses 'age' column	Schema updated, column removed
7	Use model	N/A	Schema no longer has 'age' column	Accessing 'age' attribute will fail
8	Exit	N/A	No further changes	Process complete

💡 All migrations run, database schema updated accordingly

Variable Tracker

Variable	Start	After Step 3	After Step 6	Final
users table columns	id, name, email	id, name, email, age	id, name, email	id, name, email

Key Moments - 3 Insights

Why doesn't the database schema change immediately after generating the migration file?

Because generating the migration only creates a file; the schema changes only after running the migration (see execution_table step 3).

What happens if you try to access a removed column in the model?

Accessing a removed column will cause errors because the database no longer has that column (see execution_table step 7).

Why do we specify the column type when removing a column?

Specifying the type helps Rails know exactly which column to remove, ensuring safe migration (see execution_table step 5).

Visual Quiz - 3 Questions

Look at the execution table, at which step is the 'age' column actually added to the database?

A Step 2

B Step 3

C Step 4

D Step 1

💡 Hint

Check the 'Database Schema Change' column in the execution_table rows

According to the variable tracker, what columns does the users table have after step 6?

A id, name, email, age

B id, name, email, age, created_at

C id, name, email

D age only

💡 Hint

Look at the 'After Step 6' column in variable_tracker

If you forgot to run the migration after editing the migration file, what would happen?

A The schema would remain unchanged

B The database schema would update automatically

C The migration file would run on its own

D Rails would throw an error immediately

💡 Hint

Refer to execution_table steps 2 and 3 to see when schema changes

Concept Snapshot

Adding and removing columns in Rails:
- Generate a migration file with 'rails generate migration'
- Edit the migration to add_column or remove_column
- Run 'rails db:migrate' to update the database
- Use the updated model with new or removed columns
- Always specify column type when removing columns

Full Transcript

In Rails, to add or remove columns from a database table, you first generate a migration file. This file is like a recipe for changes but does not affect the database until you run it. You then edit the migration file to specify adding or removing a column, including the column name and type. Running the migration applies these changes to the database schema. After that, your Rails model reflects the updated schema, allowing you to use the new or removed columns in your code. Remember, generating a migration file alone does not change the database; running the migration is necessary. Also, when removing columns, specifying the column type helps Rails safely identify the column to remove.