module ViewModel::ActiveRecord::ControllerBase

def viewmodel_class=(type)
  if instance_variable_defined?(:@viewmodel_class)
    raise ArgumentError.new("ViewModel class for Controller '#{self.name}' already set")
  end
  unless type < ViewModel
    raise ArgumentError.new("'#{type.inspect}' is not a valid ViewModel")
  end
  @viewmodel_class = type
end