A normal table¶

class CreateRobots < ActiveRecord::Migration
  def self.up
    create_table :robots do |t|
      t.string :name
      t.integer :energy_level
      t.text :biography
      t.timestamps
    end
  end

  def self.down
    drop_table :robots
  end
end

A join table¶

When creating a has_and_belongs_to_many relationship, the join table that is used should not have an id column. Instead, there are id columns for each of the foreign keys in the relationship. In order to prevent the automatic id column from getting created, pass :id => false to create_table.

class CreateMemberships < ActiveRecord::Migration
  def self.up
    create_table :memberships, :id => false do |t|
      t.column :group_id,   :integer
      t.column :user_id,    :integer
    end
  end

  def self.down
    drop_table :memberships
  end
end