vendor/rails/railties/doc/guides/source/migrations/creating_a_migration.txt

   1 == Creating A Migration ==
   2
   3 === Creating a model ===
   4
   5 The model and scaffold generators will create migrations appropriate for adding a new model. This migration will already contain instructions for creating the relevant table. If you tell Rails what columns you want then statements for adding those will also be created. For example, running
   6
   7 `ruby script/generate model Product name:string description:text` will create a migration that looks like this
   8
   9 [source, ruby]
  10 -----------------------
  11 class CreateProducts < ActiveRecord::Migration
  12   def self.up
  13     create_table :products do |t|
  14       t.string :name
  15       t.text :description
  16
  17       t.timestamps
  18     end
  19   end
  20
  21   def self.down
  22     drop_table :products
  23   end
  24 end
  25 -----------------------
  26
  27 You can append as many column name/type pairs as you want. By default `t.timestamps` (which creates the `updated_at` and `created_at` columns that
  28 are automatically populated by Active Record) will be added for you.
  29
  30 === Creating a standalone migration ===
  31 If you are creating migrations for other purposes (for example to add a column to an existing table) then you can use the migration generator:
  32
  33 `ruby script/generate migration AddPartNumberToProducts`
  34
  35 This will create an empty but appropriately named migration:
  36
  37 [source, ruby]
  38 -----------------------
  39 class AddPartNumberToProducts < ActiveRecord::Migration
  40   def self.up
  41   end
  42
  43   def self.down
  44   end
  45 end
  46 -----------------------
  47
  48 If the migration name is of the form AddXXXToYYY or RemoveXXXFromY and is followed by a list of column names and types then a migration containing
  49 the appropriate add and remove column statements will be created.
  50
  51 `ruby script/generate migration AddPartNumberToProducts part_number:string`
  52
  53 will generate
  54
  55 [source, ruby]
  56 -----------------------
  57 class AddPartNumberToProducts < ActiveRecord::Migration
  58   def self.up
  59     add_column :products, :part_number, :string
  60   end
  61
  62   def self.down
  63     remove_column :products, :part_number
  64   end
  65 end
  66 -----------------------
  67
  68 Similarly,
  69
  70 `ruby script/generate migration RemovePartNumberFromProducts part_number:string`
  71
  72 generates
  73
  74 [source, ruby]
  75 -----------------------
  76 class RemovePartNumberFromProducts < ActiveRecord::Migration
  77   def self.up
  78     remove_column :products, :part_number
  79   end
  80
  81   def self.down
  82     add_column :products, :part_number, :string
  83   end
  84 end
  85 -----------------------
  86
  87 You are not limited to one magically generated column, for example
  88
  89 `ruby script/generate migration AddDetailsToProducts part_number:string price:decimal`
  90
  91 generates
  92
  93 [source, ruby]
  94 -----------------------
  95 class AddDetailsToProducts < ActiveRecord::Migration
  96   def self.up
  97     add_column :products, :part_number, :string
  98     add_column :products, :price, :decimal
  99   end
 100
 101   def self.down
 102     remove_column :products, :price
 103     remove_column :products, :part_number
 104   end
 105 end
 106 -----------------------
 107
 108 As always, what has been generated for you is just a starting point. You can add or remove from it as you see fit.
 109