Rails Migrations Cheatsheet

Table methods

change_table

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

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

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_options}

drop_table

Destroys the specified table.

    drop_table :table_name

rename_table

Renames the specified table.

    rename_table :old_table_name, :new_table_name

Database Mapping

example_migration.rb

    class CreateCustomers < ActiveRecord::Migration  

      def self.up
        create_table :customers, :primary_key => :customer_id, :options => "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 :orders,    :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

Rake tasks

Directory structure

• rails_root
  • db
    • schema.rb
    • migrate
      • 20080812205401_create_customers.rb
      • 20080812215424_add_photo.rb
      • 20080812235405_alter_surname.rb

Shortcut methods

Column methods

add_column

Creates a new column on the specified table.

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

{column_options}

change_column

Change the data type of the specified column

    change_column :table_name, :column_name, :new_column_type

rename_column

Renames the specified column.

    rename_column :table_name, :old_column_name, :new_column_name 

remove_column

Removes the specified column.

    remove_column :table_name, :column_name

Indexes

add_index

Creates an index for the specified column.

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

remove_index

Remove an index from the specified column.

    remove_index :table_name, :column_name

Miscellaneous methods

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)"

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

script/generate

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

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

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

    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

Comments

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