feat(users): validation flashes for batch city and referrer

app/controllers/users_controller.rb

@@ -47,6 +47,7 @@ def edit
 
   def update
     referral_code = params.dig(:user, :referrer_code)
+    current_user.batch_id = -1 if params.dig(:user, :batch_number) # Change with impossible value to trigger validation
 
     if current_user.update_referrer(referral_code) && current_user.update(user_params)
       unlock_achievements

app/models/user.rb

@@ -28,6 +28,7 @@ class User < ApplicationRecord
   validates :username, presence: true
   validates :private_leaderboard, presence: true
 
+  validate :batch_cannot_be_changed, if: :batch_id_changed?
   validate :city_cannot_be_changed_if_present, if: :city_id_changed?
   validate :referrer_cannot_be_self
 
@@ -134,6 +135,10 @@ def update_referrer(referral_code)
 
   private
 
+  def batch_cannot_be_changed
+    errors.add(:batch, "can't be changed")
+  end
+
   def city_cannot_be_changed_if_present
     errors.add(:city, "can't be changed") if city_id_was.present?
   end