class ContainerTemplate < ApplicationRecord
  include CustomAttributeMixin
  include CustomActionsMixin

  belongs_to :ext_management_system, :foreign_key => "ems_id"
  belongs_to :container_project
  has_many :container_template_parameters, :dependent => :destroy
  has_many :labels, -> { where(:section => "labels") },
           :class_name => "CustomAttribute",
           :as         => :resource,
           :dependent  => :destroy

  serialize :objects, Array
  serialize :object_labels, Hash

  acts_as_miq_taggable

  def instantiate(_params, _project_name)
    raise NotImplementedError, _("instantiate must be implemented in a subclass")
  end
end

class ContainerTemplate < ApplicationRecord
  include CustomAttributeMixin
  include CustomActionsMixin

  belongs_to :ext_management_system, :foreign_key => "ems_id"
  belongs_to :container_project
  has_many :container_template_parameters, :dependent => :destroy
  has_many :labels, -> { where(:section => "labels") },
           :class_name => "CustomAttribute",
           :as         => :resource,
           :dependent  => :destroy

  serialize :objects, Array
  serialize :object_labels, Hash

  acts_as_miq_taggable
end

--- 
+++ 
@@ -14,4 +14,8 @@
   serialize :object_labels, Hash
 
   acts_as_miq_taggable
+
+  def instantiate(_params, _project_name)
+    raise NotImplementedError, _("instantiate must be implemented in a subclass")
+  end
 end
Add instantiate interface method in the base ContainerTemplate model.
