Rails Migrations Cheatsheet

Methods

add_column

Creates a new column on the specified table.

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

• :null => true or false - if false, the underlying column has a NOT NULL constraint added by the database engine
• :limit => size - set a limit on the size of the field
• :default => value - set a default value for the column

add_index

Creates an index for the specified table, the name of which defaults to table_column_index

    add_index :table_name, :column_name, :unique => true, :name => "chosen_index_name"

change_column

Change the data type of the specified column

    change_column :table_name, :column_name, :new_type, { options as add_column }

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, following the same format as add_column.

    create_table :table_name, { options } do |t|
      t.column :column_name, :column_type, :options
    end

• :force => true - forces drop of an existing table of the same name before creation the new one
• :temporary => true - creates a temporary table, one that goes away when the application disconnects from the database
• :id => false - defines a table with no primary key, for example when you need to define a join table
• :primary_key => :new_primary_key_name - 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
• :options => "" - lets you specify options to your underlying database, e.g. "auto_increment = 10000". Will lose default "ENGINE=InnoDB statement".

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

rename_table

Renames the specified table.

    rename_table :new_table_name, :old_table_name

rename_column

Renames the old_column_name to new_column_name

    rename_column :old_column_name, :new_column_name

rename_table

Renames the specified table.

    rename_table :new_table_name, :old_table_name

remove_index

Remove an index for the specified table.

    remove_index :table_name, :column_name

Database Mapping

Example migration file

    class CreateCustomers < ActiveRecord::Migration

    def self.up
        # Create "Customers" table
        create_table :customers, :primary_key => :customer_id, :options => "auto_increment = 10000" do |t|
          # Add columns to "Customers" table
          t.column :customer_id, :integer
          t.column :name,        :string,    :limit => 30, :null => false
          t.column :age,         :integer
          t.column :premium,     :boolean,   :default => 0
          t.column :photo,       :binary,    :limit => 2.megabytes
          t.column :thumbnail,   :binary,    :limit => 256.kilobytes
          t.column :dob,         :date,      :null => false
          t.column :created_at,  :timestamp
          t.column :notes,       :text,      :default => "No notes recorded"
      end
      # Add "surname" column to "Customers" table 
      add_column :customers, :surname,   :string,   :limit => 50
      # Add "price" column to "Orders" table
      add_column :orders,    :price,     :decimal,  :precision => 8, :scale => 2
      # Create a record on the "Customers" table
      Customer.create :name => "David", :surname => "Smith", :age => "32", :premium => "1", :notes => "One of our top customers!"
    end

    def self.down 
      # Delete the "Customers" table
        drop_table :customers
      end
    end

Rake tasks

Directory structure

• rails_root
  • db
    • schema.rb
    • migrate
      • 001_create_customers
      • 002_add_photo
      • 003_alter_surname

Testing a new section.

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