1.Using Rails Migration

1.1 Rails Migration proprties

Rails Migration is used to feed the database schema. It: 
create_table(name, options)
drop_table(name)
rename_table(old_name, new_name)
add_column(table_name, column_name, type, options)
rename_column(table_name, column_name, new_column_name)
change_column(table_name, column_name, type, options)
remove_column(table_name, column_name)
add_index(table_name, column_name, index_type)
remove_index(table_name, column_name)

and support all the basic data types: 
string, text, integer, float, datetime, 
timestamp, time, date, binary and boolean:

1.2 Creating the migrations

a. Create tables:
The related syntax is as follows:


C:\ruby\application> ruby script/generate migration table_name


This order creates the file db/migrate/001_table_name.rb. 

Now, we will create two migrations tables "hydrocarbons" 
and "groups" for the two set organics or classes Hydrocarbon 
and Group. Migrations requires names with lower case and 
in plural.


C:\ruby\organics> ruby script/generate migration hydrocarbons

C:\ruby\organics> ruby script/generate migration groups


We get:
C:\ruby\organics>ruby script/generate migration hydrocarbons
      exists  db/migrate
      create  db/migrate/001_hydrocarbons.rb

C:\ruby\morganics>ruby script/generate migration groups
      exists  db/migrate
      create  db/migrate/002_groups.rb

C:\ruby\organics>


b. Configure the corresponding tables

In db/migrate subdirectory of the related application 
"organics"; edit the two fresh Ruby created files:
001_create_hydrocarbons.rb and 002_create_groups.rb

001_create_hydrocarbons.rb
Change this file to become as follows:


class Hydrocarbons < ActiveRecord::Migration
	def self.up
	create_table :hydrocarbons do |table|
	  # id is created for automatically.
	table.column :compound, :string, :limit => 30, :null => false
	table.column :nbCarbons, :integer
	table.column :formula, :string, :limit => 25, :null => false
	table.column :image, :binary
	table.column :group_id, :integer
	table.column :description, :text
	table.column :created_at, :timestamp
	end
	end

	def self.down
	drop_table :hydrocarbons
	end
	end


  def self.down

    drop_table :hydrocarbons

  end

end



002_create_groups.rb
Change this file to become as follows:


	class Groups < ActiveRecord::Migration
	def self.up
	create_table :groups do |table|
	table.column :functional, :string
	end
	Group.create :functional => "Alcohol"
	Group.create :functional => "Esters"
	Group.create :functional => "Ethers"
	Group.create :functional => "Ketones"
	Group.create :functional => "Amines"
	Group.create :functional => "Aldehydes"
	Group.create :functional => "Carboxylic_Acides"
	end

	def self.down
	drop_table :groups
	end
	end


2 Run the migrations
We need to configure those two files against the 
related set databases. This is done by the following 
command line:

C:\ruby\organics> rake db:migrate


The program Rake comes with Ruby. "rake migrate" is an 
important Rails property. It
- updates the change in the shema database.
- updates the the database against a new release.
- If it doesn't already exist, creates "schema_info" 
file to track and set a current version.