Rails migration

It's my first time to touch Rails migration, so I'll write it down briefly as a memorandum of study.

About migration

Migration is a convenient way to make continuous changes to your database schema unified and easy.

Create a migration file

Prepare Company table as a sample

Column name Data type
id integer
name string
description text
created_at datetime
updated_at datetime
$ rails g migration CrateCompany

Active Record automatically displays the migration execution order with the time stamp of the file name. The created file is saved in the db / migrate directory.

Created file

class CrateCompany < ActiveRecord::Migration[5.2]
	def change

ActiveRecord :: Migration [] contains the version.

You can use up and down instead of change. In the up method, write the contents when migrating, and in the down method, write the contents when rolling back.

class CrateCompany < ActiveRecord::Migration[5.2]
	def up

	def down

Make changes

Create a migrate file and type rails db: migrate to run it. Also, to undo the last change, rails db: rollback

Create a table

class CrateCompany < ActiveRecord::Migration[5.2]
    def change 
     create_table :companies do |t|
    t.string :name
      t.text :description

If you type this command, it will automatically create something like the above

$ rails g migration CrateCompany name:string description:text

Change table

class ChangeTableCompany < ActiveRecord::Migration[5.2]
    def change 
     change_table :companies do |t|
     t.remove :name
     t.string :root_number
     t.index :root_number
     t.rename :description, :description_note

The migration above removes the name column, creates a string column, root_number, and adds the index to it. And finally, the description column is renamed.

Change column

change_column :companies, :root_number, :text

Changed the root_number column of the model name to text.

Note that change_column cannot be rolled back with the change method. The change method does not support it. The migration definitions supported by change are

add_column add_foreign_key add_index add_reference add_timestamps change_column_default (specifying: from and: to cannot be omitted) change_column_null create_join_table create_table disable_extension drop_join_table drop_table (must pass a block) enable_extension remove_column (type must be specified) remove_foreign_key (must specify second table) remove_index remove_reference remove_timestamps rename_column rename_index rename_table

If you want to roll back, you can use change_column if it is an up or down method.

class ChangeColumnCompany < ActiveRecord::Migration[5.2]
    def up
     change_column :companies, :root_number, :text

    def down
     change_column :companies, :root_number, :string

I don't know much about migration yet, but I want to learn it.

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