Active Record has_one is usually bad

app__models__account.rb

class Account < ActiveRecord::Base
  belongs_to :supplier
end

app__models__supplier.rb

class Supplier < ActiveRecord::Base
  has_one :account
end

db__migrate__00-create-suppliers.rb

class CreateSuppliers < ActiveRecord::Migration
  def change
    create_table :suppliers do |t|
      t.string :name
      t.timestamps null: false
    end

    create_table :accounts do |t|
      t.integer :supplier_id
      t.string :account_number
      t.timestamps null: false
    end

    add_index :accounts, :supplier_id
  end
end