feat(subscription): run validations when needed

* add optional params for user.
* accept nested attrs for both :user and :plan

app/models/subscription.rb

@@ -1,9 +1,16 @@
 # frozen_string_literal: true
 
 class Subscription < ApplicationRecord
-  belongs_to :user
+  belongs_to :user, optional: true
   belongs_to :plan
 
-  validates :user_id, :plan_id, presence: true
-  validates :user_id, uniqueness: { scope: %i[plan_id active] }
+  accepts_nested_attributes_for :user
+  accepts_nested_attributes_for :plan
+
+  validates :plan_id, presence: true
+  validates :user_id, uniqueness: { scope: %i[plan_id active] }, if: :user?
+
+  def user?
+    !user_id.blank?
+  end
 end

spec/models/subscriptions_spec.rb

@@ -15,7 +15,6 @@
   end
 
   describe 'validations' do
-    it { is_expected.to validate_presence_of(:user_id) }
     it { is_expected.to validate_presence_of(:plan_id) }
     it { is_expected.to validate_uniqueness_of(:user_id).scoped_to(:plan_id, :active).ignoring_case_sensitivity }
   end