Table of Contents

1. Table methods

1.1. change_table

Provides a block that enables you to alter columns on an existing table using various shortcut methods.

change_table :table_name do |t| t.change :column_name, :new_column_type t.remove :column_name end

1.2. create_table

Creates a table on the database. Creates a table called :table_name and makes the table object available to a block that can then add columns to it by specifying column types or utilising shortcut methods such as using belongs_to to specify foreign keys…

create_table :table_name, {table_options} do |t| t.string :name, {column_options} end

Table 1.2.1 {table_options}

Key	Value	Description
:force	true or false	if true, forces drop of an existing table of the same name before creation the new one
:temporary	true or false	if true, creates a temporary table, one that goes away when the application disconnects from the database
:id	true or false	if false, defines a table with no primary key, for example when you need to define a join table
:primary_key	:symbol	overrides the default name of :id for the primary column. Use this to specify the name of the column in the database that Rails will use to store the primary key
ptions	"string"	pass raw options to your underlying database, e.g. auto_increment = 10000. Note that passing options will cause you to lose the default ENGINE=InnoDB statement

1.3. drop_table

Destroys the specified table.

drop_table :table_name

1.4. rename_table

Renames the specified table.

rename_table   ld_table_name, :new_table_name

2. example_migration.rb

class CreateCustomers < ActiveRecord::Migration def self.up create_table :customers, :primary_key => :customer_id,   ptions => "auto_increment = 10000" do |t| t.integer :customer_id t.string :name, :limit => 30, :null => false t.integer :age t.boolean :premium, :default => 0 t.binary :photo, :limit => 2.megabytes t.timestamps t.text :notes, :default => "No notes recorded" end add_column :customers, :surname, :string, :limit => 50 add_column   rders, :price, :decimal, :precision => 8, :scale => 2 Customer.create :name => "David", :surname => "Smith", :age => "32", :premium => "1", :notes => "One of our top customers!" end def self.down drop_table :customers end end

3. Database Mapping

4. Rake tasks

db:create

db:create:all

Creates a single database specified in config/databases.yml for the current RAILS_ENV or creates all the databases

db:drop

db:drop:all

Drops a single database specified in config/databases.yml for the current RAILS_ENV or drops all the databases

db:fixtures:load

Load fixtures from test/fixtures into the current environment’s database

db:migrate

Run all unapplied migrations

db:migrate:up

db:migrate:down

Move forward to the next migration, or back to the previous migration

db:migrate VERSION=18

Migrate database to specific version

db:migrate RAILS_ENV=production

Use migrations to recreate tables in the testing or production databases

db:schema:dump

Create a db/schema.rb file that can be portably used against any database supported by ActiveRecord

db:schema:load

Load a schema.rb file into the database

db:sessions:create

Create a sessions table for use with CGI::Sessions::ActiveRecordStore

db:sessions:clear

Clear the sessions table

db:structure:dump

Dump database structure to SQL file

db:reset

Drops the database, creates the database and then runs migrations against the database. Takes a VERSION argument as well as RAILS_ENV

db:rollback STEP=4

Takes a STEP argument to determine how many version to rollback, the default being one version

db:test:prepare

Clone your database structure into the test database

db:version

Tells you the current version your database is at

5. Directory structure

RAILS_ROOT /db + schema.rb /migrate + 20081204220440_remove_sections_table.rb + 20081204220455_add_customers_table.rb

6. Shortcut methods

t.column t.change t.rename t.remove t.change_default t.references t.remove_references t.belongs_to t.remove_belongs_to t.timestamps t.index t.remove_index

7. Column methods

7.1. add_column

Creates a new column on the specified table.

add_column :table_name, :column_name, :column_type, {column_options}

Table 7.1.1 {column_options}

7.2. change_column

Change the data type of the specified column

change_column :table_name, :column_name, :new_column_type

7.3. rename_column

Renames the specified column.

rename_column :table_name,   ld_column_name, :new_column_name 

7.4. remove_column

Removes the specified column.

remove_column :table_name, :column_name

8. Indexes

8.1. add_index

Creates an index for the specified column.

add_index :table_name, :column_name, :unique => true

8.2. remove_index

Remove an index from the specified column.

remove_index :table_name, :column_name

9. Miscellaneous methods

9.1. execute

Takes a single string identifying a valid SQL command to execute directly.

execute "alter table line_items add constraint fk_line_item_products foreign key (product_id) references products(id)"

9.2. IrreversibleMigration

Use in the down method of a migration file to raise an exception when the up methods of the same migration file can not be reversed, e.g. changing a column type from :integer to :string.

raise ActiveRecord::IrreversibleMigration

10. script/generate

script/generate migration new_migration_filename field_name:column_type name:string age:integer date_of_birth:date

11. Loading fixtures

Fixtures contain data which can be loaded into your database using migrations. For example, to load data into a table named customers…

• Create a directory, db/migrate/data
• Create a file, customers.yml, inside db/migrate/data
• Generate a new migration file: ruby script/generate migration load_customers_data
• Edit it to load data from the customers.yml file into your customers table

11.1. customers.yml

melissa: name: Melissa age: 18 david: name: David age: 23 

11.2. migration.rb

require 'active_record/fixtures' class LoadCustomerData def self.up down directory = File.join(File.dirname(__FILE__), "data") Fixtures.create_fixtures(directory, "customers") end def self.down Customer.delete_all end end