upto 3.11: added option to join to beerclub

app/models/membership.rb

@@ -1,4 +1,12 @@
 class Membership < ApplicationRecord
   belongs_to :user
   belongs_to :beerclub
+
+  validate :user_not_in_club
+
+  def user_not_in_club
+    if not Membership.find_by(user_id:user_id, beerclub_id:beerclub_id).nil?
+      errors.add :user_id, "is already in the beerclub"
+    end
+  end
 end