If we have has_one relation between two models then we need to declare belongs_to relation in one model and has_one relation in another model. belongs_to will be declared in the model where we will have the foreign_key and has_one in the other model . Generally has_one relation is used where we need to define possession like User has one Wallet.

We can add the relation like this:

class User < ActiveRecord::Base
  has_one :wallet
end

class Wallet < ActiveRecord::Base
  belongs_to :user
end

The corresponding migration might look like this:

class CreateUsers < ActiveRecord::Migration
  def change
    create_table :users do |t|
      t.string  :name
      t.timestamps
    end

  end
 end
class CreateWallets < ActiveRecord::Migration
  def change
    create_table :wallets do |t|
      t.integer :user_id
      t.integer :amount
      t.timestamps
    end
  end
 end

Instead of t.integer :user_id we can also write t.references :user