Raise an exception if required methods for ViewableModel are not implemented [SCI-3436]

app/models/concerns/viewable_model.rb

@@ -3,14 +3,23 @@
 module ViewableModel
   extend ActiveSupport::Concern
 
+  included do
+    has_many :view_states, as: :viewable, dependent: :destroy
+  end
+
   # This module requres that the class which includes it implements these methods:
   # => default_view_state, returning hash with default state representation
   # => validate_view_state(view_state), custom validator for the state hash
 
-  included do
-    has_many :view_states, as: :viewable, dependent: :destroy
+  def default_view_state
+    raise NotImplementedError, 'default_view_state should be implemented!'
   end
 
+  def validate_view_state(view_state)
+    raise NotImplementedError, 'validate_view_state(view_state) should be implemented!'
+  end
+
+
   def current_view_state(user)
     state = view_states.where(user: user).take
     state || view_states.create!(user: user, state: default_view_state)