How to Create Migration in Rails: Step-by-Step Guide

To create a migration in Rails, use the rails generate migration MigrationName command in your terminal. This creates a migration file where you define changes to your database schema, such as adding or removing tables or columns.

Syntax

The basic syntax to create a migration in Rails is:

rails generate migration MigrationName [field:type field:type ...]

Here, MigrationName is a descriptive name for the change, and optional field:type pairs define columns to add or modify.

bash

rails generate migration AddTitleToPosts title:string

Example

This example creates a migration to add a title column of type string to the posts table.

After running the command, edit the generated migration file if needed, then run rails db:migrate to apply changes.

bash

rails generate migration AddTitleToPosts title:string

# Migration file generated (timestamp_add_title_to_posts.rb):
# class AddTitleToPosts < ActiveRecord::Migration[7.0]
#   def change
#     add_column :posts, :title, :string
#   end
# end

rails db:migrate

Output

== 20240601000000 AddTitleToPosts: migrating ===============================
-- add_column(:posts, :title, :string)
   -> 0.0012s
== 20240601000000 AddTitleToPosts: migrated (0.0013s) ==============================

Common Pitfalls

• Using vague migration names like CreateTable instead of descriptive names like CreateUsersTable.
• Forgetting to run rails db:migrate after creating the migration.
• Editing migration files after they have been run without rolling back first.
• Not specifying the correct data types for fields.

bash

### Wrong way:
rails generate migration CreateTable

### Right way:
rails generate migration CreateUsersTable name:string email:string

Quick Reference

Here is a quick reference for common migration commands:

Command	Description
rails generate migration MigrationName	Creates a new migration file
rails db:migrate	Runs pending migrations to update the database
rails db:rollback	Reverts the last migration
rails generate migration AddFieldToTable field:type	Adds a new column to a table
rails generate migration RemoveFieldFromTable field:type	Removes a column from a table

Key Takeaways

Use rails generate migration MigrationName to create migration files.

Name migrations clearly to describe the database change.

Always run rails db:migrate to apply migrations.

Avoid editing migrations after running them without rollback.

Specify correct field types when adding or modifying columns.