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# Migrations

### Automatically make migrations

    $ rails generate migration RemovePartNumberFromProducts part_number:string
    $ rails generate migration remove_part_number_from_products part_number # rails assumes string if not type given - and you can use snake_case

    $ rails generate migration AddNameToWidgets name:string
    $ rails g migration add_name_to_widgets name:string # you can use the short cut 'g' instead of generate - they both do the same thing

### Run migrations

    $ rake db:migrate

### Creating tables

    create_table :users do |t|
      t.string :name
      t.text   :description

      t.primary_key :id
      t.string      :title
      t.text        :description
      t.integer     :games_count
      t.float       :lol
      t.decimal     :price
      t.decimal     :price, :precision => 2, :scale => 10
      t.datetime    :expiration
      t.timestamp   :time_in
      t.time        :time_in
      t.date        :expiry
      t.binary      :image_data
      t.boolean     :is_admin
    end

    # Options:
      :null (boolean)
      :limit (integer)
      :default

### Operations

    add_column    :users, :first_name, :string
    remove_column :users, :first_name, :string

    change_column :users, :first_name, :text

    change_column_default :users, :admin, nil
    change_column_null    :users, :email, false # adds NOT NULL constraint

    create_table
    change_table
    drop_table

    add_column
    change_column
    rename_column
    remove_column

    add_index
    remove_index

### Use models

    class AddFlagToProduct < ActiveRecord::Migration
      class Product < ActiveRecord::Base
      end

      def change
        add_column :products, :flag, :boolean
        Product.reset_column_information
        reversible do |dir|
          dir.up { Product.update_all flag: false }
        end
      end
    end

### Associations

    t.references :category   # kinda same as t.integer :category_id

    # Can have different types
    t.references :category, polymorphic: true

### Auto-Add/remove columns

    $ rails generate migration RemovePartNumberFromProducts part_number:string

### Indices

    # Simple
    add_index :suppliers, :name

    # Unique
    add_index :accounts, [:branch_id, :party_id], :unique => true

    # Named (:name => ...)
    add_index :accounts, [:branch_id, :party_id], :unique => true, :name => "by_branch_party"

    # Length
    add_index :accounts, :name, :name => ‘by_name’, :length => 10
    add_index :accounts, [:name, :surname], :name => ‘by_name_surname’,
      :length => {
        :name => 10,
        :surname => 15
      }

    # Sort order (no MySQL support)
    add_index :accounts, [:branch_id, :party_id, :surname],
      :order => {:branch_id => :desc, :part_id => :asc}

### In console

Use `ActiveRecord::Migration`.

    ActiveRecord::Migration.add_index :posts, :slug

### References

- http://apidock.com/rails/ActiveRecord/ConnectionAdapters/SchemaStatements/add_index

Devhints.io cheatsheets is a collection of cheatsheets I've written over the years. Suggestions and corrections? Send them in. I'm Rico Sta. Cruz. Check out my Today I learned blog for more.