init project

app/models/concerns/watchable.rb

@@ -0,0 +1,26 @@
+module Watchable
+  extend ActiveSupport::Concern
+
+  included do
+    has_many :watchers, as: :watchable, dependent: :destroy
+    has_many :watcher_users, through: :watchers, source: :user, validate: false
+
+    scope :watched_by, -> (user_id) { includes(:watchers).where(watchers: { user_id: user_id }) }
+  end
+
+  def watched?(watchable)
+    watchable.watchers.exists?(user: self)
+  end
+
+  def watch!(watchable)
+    watchable.watchers.create!(user: self)
+  end
+
+  def unwatch!(watchable)
+    obj = watchable.watchers.find_by(user: self)
+    obj.destroy! if obj.present?
+  end
+
+  module ClassMethods
+  end
+end