Rails
Database Migration
================
Based on guides.rubyonrails.org
Based on guides.rubyonrails.org
Use Ruby DSL(Domain Specific
Language) instead of SQL
File updated is db/schema.rb
Ex:
class CreateProducts
< ActiveRecord::Migration
def change
create_table :products do |t|
t.string :name
t.text :description
t.timestamps
end
end
end
t.timestamps creates the
columns created_at and updated_at.
Creating the Migration
Command:
$ rails
generate migration CommandClass
Migrations are created
in db/migrate as the name:
YYYMMDDHHMMSS_NAME_CLASS(table).rb
Ex:
$ rails
generate migration AddPartNumberToProducts
generates in
a file YYYMMDDHHMMSS_add_details_to_products.r:
class
AddPartNumberToProducts < ActiveRecord::Migration
def change
end
end
Use conventions:
CreateXXX
$
rails generate migration CreateProducts name:string part_number:string
-generates:
class
CreateProducts < ActiveRecord::Migration
def change
create_table :products do |t|
t.string :name
t.string :part_number
end
end
end
AddXXXToYYY
$
rails generate migration AddPartNumberToProducts part_number:string:index
price:decimal
generates:
class
AddPartNumberToProducts < ActiveRecord::Migration
def change
add_column :products, :part_number, :string
add_index :products, :part_number
add_column :products, :price, :decimal
end
end
RemoveXXXFromYYY
$
rails generate migration RemovePartNumberFromProducts part_number:string
-generates
class
RemovePartNumberFromProducts < ActiveRecord::Migration
def change
remove_column :products, :part_number,
:string
end
end
Including reference to
another Class
$ rails
generate migration AddUserRefToProducts user:references
-generates
class
AddUserRefToProducts < ActiveRecord::Migration
def change
add_reference :products, :user, index: true
end
end
- This
migration will create a user_id column and appropriate index.
Joint table
$ rails g
migration CreateJoinTableCustomerProduct customer product
-generates
class
CreateJoinTableCustomerProduct < ActiveRecord::Migration
def change
create_join_table :customers, :products do
|t|
# t.index [:customer_id, :product_id]
# t.index [:product_id, :customer_id]
end
end
end
Model Generators
When generates a model,
rails will also generate a migration file
$ rails generate model
Product name:string description:text
- generates
class
CreateProducts < ActiveRecord::Migration
def change
create_table :products do |t|
t.string :name
t.text :description
t.timestamps
end
end
end
Add Type modifier
- limit Sets the maximum
size of the string/text/binary/integer fields
- precision Defines the
precision for the decimal fields
- scale Defines the
scale for the decimal fields
- polymorphic Adds a
type column for belongs_to associations
- null Allows or
disallows NULL values in the column.
Ex:
$ rails
generate migration AddDetailsToProducts price:decimal{5,2}
supplier:references{polymorphic}
- generates
class
AddDetailsToProducts < ActiveRecord::Migration
def change
add_column :products, :price, :decimal,
precision: 5, scale: 2
add_reference :products, :supplier,
polymorphic: true, index: true
end
end
Parts of Migration file
Create table
create_table
:products, options: "ENGINE=BLACKHOLE" do |t|
t.string :name, null: false
end
Join Table
create_join_table
:products, :categories, column_options: {null: true}
-
Creates a new table categories_products with 2 columns(category_id, product_id)
with the option that column can be null.
create_join_table
:products, :categories, table_name: :categorization
-
Set the name of the table_name
Add Index which is not
created by default
create_join_table
:products, :categories do |t|
t.index :product_id
t.index :category_id
end
Change Table
Change table,
including/removing/renaming and so on
Ex:
change_table
:products do |t|
t.remove :description, :name #remove column description and name
t.string :part_number #Creates par_number column
t.index :part_number #creates part_number index
t.rename :upccode, :upc_code
end
Executing a SQL directry
Products.connection.execute('UPDATE
`products` SET `price`=`free` WHERE 1')
Methods
They can be used when
the migration can be reverted
add_column
add_index
add_reference
add_timestamps
create_table
create_join_table
drop_table (must supply
a block)
drop_join_table (must
supply a block)
remove_timestamps
rename_column
rename_index
remove_reference
rename_table
Reversible
Used in complex
migrations where Active Records doesn't know to revese. Using reverse, it is
possible to specify what to do in a migration.
Ensure that the actions
are executed in the right order.
Up/Down methods
Old style of migration
intead of change method.
Up
should
describe the transformation you'd like to make to your schema
Down
should revert
the transformations done by the up method
Ex:
Create a
table in up and delete it in down
Running migration
$ rake db:migrate
VERSION=20080906120000
rollback 3 steps
$ rake
db:rollback STEP=3
roolback 3 steps and
redo the migration
$ rake
db:migrate:redo STEP=3
Reset database
Drop the
table, recreate and load the current schema
$ rake
db:reset
- If a
migrantion can't be rolledback, the command doesn't work.
Running migration of
different environment
$ rake
db:migrate RAILS_ENV=test
Output
say -> print a
message
say
'something'
suppress_messages ->
avoid any message in a block
ppress_messages
{add_index :products, :name}
say
"and an index!", true
say_with_time
Wait some
time to say something.
