Generating Active Record Migrations
Why
Migration in Rails is a tool that allows the developer to use Ruby code to change an application’s database schema. Instead of using SQL scripts, we use Ruby code, which is database independent. Active Record will also update your db/schema.rb file to match the up-to-date structure of your database.
What
Migrations are stored as files in the db/migrate directory. In each migration file defines a migration class, which prefixed by a timestamp of the current date and time.
Rails provide a tool for you to generate migration in command-line. Usage:
bin/rails generate(g) migration MIGRATION_NAME [field[:type][:index] field[:type][:index]] [options]
Notice, fiels parameters decide what columns to add or remove.
How
Creating New Tables
MIGRATION_NAME is create_{table_name} followed by a list of column names and types.
rails generate migration CreateProducts name:string part_number:string
Adding Columns
MIGRATION_NAME is add_{column1}_{column2}_to_{table_name} followed by a list of column names and types. Notice, column names should match.
rails g migration add_name_part_number_to_products name:string part_number:string
Removing Columns
MIGRATION_NAME is remove_{column1}_{column2}_to_{table_name} followed by a list of column names and types. Types are optional but can be useful for reverting.
Creating Standalone Migrations
You can create an empty migration, which defines an empty change method.
rails g migration {any_name_you_want}
Model Generators
The model, resource, and scaffold generators will create migrations appropriate for adding a new model.
rails generate model Product name:string description:text
Where
This is database-related operation. Please distinguish from model (ActiveRecord) behaviors.
When
Each time before you want to change database schema, like adding new tables, changing or deleting existing tables.