1. Agile Web Development with Ruby and Rails
Prof. Paul Krause
Lecture 7
Migrations and Active Record

2. Objectives for today A Look at Migrations
From Migrations to Active Record
Rails’ Object-Relation Mapping
More on Rails Controllers

4. 20091030111023_add_price_to_product
class AddPriceToProduct < ActiveRecord::Migration
def self.up
add_column :products, :price, :decimal
end
def self.down
remove_column :products, :price

6. 20091030111023_add_price_to_product
class AddPriceToProduct < ActiveRecord::Migration
def self.up
add_column :products, :price, :decimal,
:precision => 8, :scale => 2, :default => 0
end
def self.down
remove_column :products, :price

9. Two ways of Generating Migrations
Create a Model
E.g. generating a model called Discount will also generate a migration:
dddddddddddddd_create_discounts.rb
Generate a migration on its own
E.g. Generate/Migration add_price_column
You then need to ask Rails to run a “Rake” task to migrate to the new version

10. Anatomy of a migration
class AddPriceToProduct < ActiveRecord::Migration
def self.up
add_column :products, :price, :decimal,
:precision => 8, :scale => 2, :default => 0
end

11. Anatomy of a migration
class AddPriceToProduct < ActiveRecord::Migration
def self.up
add_column :products, :price, :decimal,
:precision => 8, :scale => 2, :default => 0
end
def self.down
remove_column :products, :price

12. Column Types add_column :orders, :e-mail, :string
The third parameter specifies the type of the database column.
But databases typically don’t have :string as a type?
Rails insulates you from the underlying database by using “logical types”
E.g. :string type will generate a column of type varchar(255) in MySQL

13. Column Options :null => true or false :limit => size
:default => value
N.B. the value is determined at the time of application of the migration:
add_column :orders, :placed_at, :datetime, :default => Time.now
decimal columns also support the options :precision and :scale

14. Summary of migration types
Create tables
Add columns
Rename columns
Change the type, or options, of columns
Rename tables

15. Warning about renaming tables
If you rename a table (e.g. order_histories -> order_notes), you will also need to rename the associated model (e.g. OrderHistory -> OrderNote)
Now, suppose you wish to drop the database, and then reapply all migrations?
Rerunning the migration to create the table order_histories will generate an exception, as the associated model has been renamed!
There are ways around this, but be aware that Rails migrations are not foolproof...

16. Primary Keys You will not normally explicitly define a primary key
Rails assumes:
every table has a numeric (integer) primary key (called id)
Rails ensures
the value of this column is unique for each row added to the database
You can change the name of the primary key:
create_table :tickets, :primary_key => :number do |t| ..
But it is not a good idea to change the type from an integer

17. Primary Keys You will not normally explicitly define a primary key
Rails assumes:
every table has a numeric (integer) primary key (called id)
Rails ensures
the value of this column is unique for each row added to the database
You can change the name of the primary key:
create_table :tickets, :primary_key => :number do |t| ..
But it is not a good idea to change the type from an integer

18. Tables with no Primary Key
The most common case for this is a join table
Need to explicitly tell Rails not to generate a primary key:
create_table :authors_books, :id => false do |t|
t.integer :author_id, :null => false
t.integer :book_id, :null => false
end

19. From Migrations to Active Record

20. Active Record This is Rails’ Object-Relational mapping:
Tables map to classes;
Rows map to objects;
Columns map to object attributes.
The “Rails way” is to use sensible defaults to minimise the configuration needed

21. Simple Example that Does a Lot...
ActiveRecord::Base.establish_connection(:adapter => “jdbcmysql”, :database => “development”
class Order < ActiveRecord::Base
end end
order = Order.find(1)
order.name = “Paul Krause”
order.save
Connect to the database
Wrap the table of Orders in the database
Find the order with a specific ID
Change the purchaser’s name
Save the result back in the database

22. More on the slides from Sun Microsystems ...