Migrations

Location

• db/migrate/
• Should be prefix with the timestamp of the creation. 20170501123345_my_migration.rb

Best pratices in Migrations

A migration is a one shot operation on the database. You must follow this recommendations:

• Default values for booleans: boolean should always be true or false
• Indexes: think about the fields you will query in the future and add indexes to them. It's critical for performances and you must have indexes for foreign keys (user_id, post_id, etc).
• Rollback: all migrations should be reversible. Create a #up and #down method on each migration.
• Never edit a migration after push: people in your team will pull your code and run the latest migrations. The code pushed will also be deployed to production. If you did a mistake or forgot something in your migration (like indexes or default values), you have to create another migration and push it.
• Data operations: If you need to migrate data structure in your database, use a migration. Confirm with the team that you can apply it and everything is ready to handle this data migration.

Code

class MyMigrationnameHere < ActiveRecord::Migration
  def change
    add_column :events, :refresh_interval, :integer
    add_column :events, :admin, :boolean, default: false

    # Normal index
    add_column :users, :project_id, :integer
    add_index :users, :project_id

    # Combine indexes:
    add_index :users, [:kind, :created_at]

    # Using GIST or GIN (PostgreSQL)
    add_index :users, :tags, using :gin
    add_index :users, :tags, using :gist
  end
end