Both ProvisioningTemplate and Ptable contain overrides of the Rails base_class method, indicating that the STI base class of ProvisioningTemplate (etc) is ProvisioningTemplate, rather than Template.

# we have to override the base_class because polymorphic associations does not detect it correctly, more details at
# http://apidock.com/rails/ActiveRecord/Associations/ClassMethods/has_many#1010-Polymorphic-has-many-within-inherited-class-gotcha
def self.base_class
  self
end
# not sure why but this changes table_name so we set it explicitly
self.table_name = 'templates'

This causes an issue with the Template model instantiation under Rails 5.0, as ActiveRecord performs casting of new models, much like Foreman::STI does. This relies on base_class returning the actual base class, else the inheritance appears inverted and causes this exception:

DomainTest#test_0008_should update total_hosts on changing primary interface domain:
ActiveRecord::SubclassNotFound: Invalid single-table inheritance type: Template is not a subclass of Ptable
    activerecord (5.0.0) lib/active_record/inheritance.rb:189:in `find_sti_class'
    activerecord (5.0.0) lib/active_record/inheritance.rb:209:in `subclass_from_attributes'
    activerecord (5.0.0) lib/active_record/inheritance.rb:58:in `new'

Removing base_class causes the problem noted in the comment above, that polymorphic associations to these models break - in fact, they search for the base class rather than the specific class. https://github.com/rails/rails/blob/v5.0.0/activerecord/lib/active_record/associations.rb#L780 notes that in actual fact, the base class should be stored in the type column for polymorphic associations, not the derivative.

This should be changed to remove the overrides and store the base class when dealing with polymorphic STI associations (both Taxonomy and Audit are associated to Template derivatives).