Rails DB migrations

Database migrations in Rails Denys Kurets

Agenda - What are database migrations; - Database migrations in Rails; - Best practices;

In software engineering, schema migration (also database migration, database change management) refers to the management of incremental, reversible changes to relational database schemas. A schema migration is performed on a database whenever it is necessary to update or revert that database's schema to some newer or older version. © Wikipedia Migrations are a feature of Active Record that allows you to evolve your database schema over time. Rather than write schema modifications in pure SQL, migrations allow you to use an easy Ruby DSL to describe changes to your tables. What are database migrations?

Flow: • Use Ruby code (Database independent); • Run rake task; • Other developer run rake task; • Run rake task during deploy Database migrations in Rails

Database migrations in Rails Migration are classes:

Database migrations in Rails • Database independent way; • db/migrate - directory; • YYYYMMDDHHMMSS_add_details_to_products.rb; • Execute method allows you to execute arbitrary SQL; • Migrations are wrapped in transaction(PostgreSQL or r SQLite3, not support MySQL*); * stackoverflow

Database migrations in Rails rails g model Product name:string description:text rails g scaffold Product name:string description:text rails g migration AddShortToProducts short:text rails g migration RemoveShortFromProducts short:text To create migration files:

Database migrations in Rails Add and Remove Columns: • AddXXXtoYYY • RemoveXXXFromYYY rails g migration AddShortToProducts short:text rails g migration RemoveShortFromProducts short:text

create_table Methods for db structure changing create_table :friends do |t| t.string :first_name t.string :last_name t.string :email t.timestamps end change_table change_table :friends do |t| t.remove :last_name, :first_name t.string :phone_number t.index :phone_number end add_column change_column :friends, :last_name, :string change_column add_column :friends, :first_name, :string add_column :friends, :last_name, :integer

Methods for db structure changing remove_column :friends, :surname remove_column rename_column :friends, :last_name, :surname rename_column remove_index :friends, :email remove_index add_index :friends, :email drop_table :friends add_index drop_table add_foreign_key :articles, :authors add_foreign_key & remove_foreign_key

● :binary ● :boolean ● :date ● :datetime ● :decimal ● :float Column Types: Database migrations in Rails ● :integer ● :primary_key ● :string ● :text ● :time ● :timestamp ● :references

Change method ● add_column ● add_foreign_key ● add_index ● add_reference ● add_timestamps ● change_column_default ● change_column_null ● create_join_table ● create_table ● disable_extension ● drop_join_table ● drop_table ● enable_extension ● remove_column ● remove_foreign_key ● remove_index ● remove_reference ● remove_timestamps ● rename_column ● rename_index ● rename_table class CreateProducts < ActiveRecord::Migration def change create_table :products do |t| t.string :name t.text :description t.timestamps null: false end end end

Database migrations in Rails rake db:migrate #up rake db:rollback #down rake db:migrate:status rake db:version #current actual migration version rake db:migrate VERSION=20151117195012 #migrate to version rake db:migrate:up VERSION=20151117195012 #migrate specific rake db:migrate:down VERSION=20151117195012 #migrate specific rake db:migrate:redo VERSION=20151117195012 #migrate down and up rake db:rollback STEP=3 #revert last 3 migrations rake db:migrate RAILS_ENV=test #run in test environment More commands:

Database migrations in Rails (How it works?)

Database migrations in Rails rake db:migrate:down VERSION=20150321134933 rake db:rollback rake db:migrate

rake db:rollback STEP=3 rake db:migrate VERSION=20151124211406 rake db:migrate:up VERSION=20151124211419 Database migrations in Rails

Init app database: Database migrations in Rails rake db:setup 1. db:create 2. db:schema:load 3. db:seed rake db:reset 1. db:drop 2. db:setup

db/schema.rb: • “...schema dumps are the authoritative source for your database schema... ” • “...two ways to dump the schema… This is set in config/application.rb by the config. active_record.schema_format setting, which may be either :sql or :ruby” Database migrations in Rails rake db:schema:dump #called by db:migrate rake db:schema:load

Output of migrations: • suppress_messages - Takes a block as an argument and suppresses any output generated by the block. • say - Takes a message argument and outputs it as is. A second boolean argument can be passed to specify whether to indent or not. • say_with_time - Outputs text along with how long it took to run its block. If the block returns an integer it assumes it is the number of rows affected. Database migrations in Rails

Best practices • “...developers should not be afraid to clear out the old migrations directory, dump a new schema, and continue on from there...”; • Never have data only migrations, or migration that change existing data; • Keep the schema.rb (or structure.sql) under version control; • Use rake db:schema:load instead of rake db:migrate to initialize an empty database; • When writing constructive migrations (adding tables or columns), use the change method instead of up and down methods; • Enforce default values in the migrations themselves instead of in the application layer; • Atomic changes;

References 1. https://en.wikipedia.org/wiki/Schema_migration 2. http://edgeguides.rubyonrails.org/active_record_migrations.html 3. https://github.com/bbatsov/rails-style-guide#migrations 4. http://stackoverflow.com/questions/10301794/difference-between-rake-dbmigrate-dbreset-and-dbschemaload 5. http://stackoverflow.com/questions/9884429/rails-what-does-schema-rb-do 6. http://stackoverflow.com/questions/686852/rolling-back-a-failed-rails-migration 7. http://www.slideshare.net/seapy/rails-database-migrations-rorlab-season-33 8. http://kottans.org/ruby-slides/public/models/#migrations-topic 9. https://gist.github.com/pyk/8569812 10. http://www.toptal.com/ruby-on-rails/top-10-mistakes-that-rails-programmers-make 11. https://robots.thoughtbot.com/referential-integrity-with-foreign-keys 12. https://www.google.com.ua/ 13. http://selectedproblems.blogspot.com/2011/09/rails-migrations-best-practices.html

"the only stupid question is the one that isn't asked"

Rails DB migrations

More Related Content

Similar to Rails DB migrations

Recently uploaded

Rails DB migrations