has_one foreign_key_on: :related fails on save

When saving a resource with a has_one relationship with foreign_key_on: :related I get an error of undefined method 'foreign_key=' for the model. I'm not sure if the intent is to change the relationship on the related items when save or not. For now I did the following patch to not update any relationships that have foreign_key_on: :related

def replace_to_one_link(relationship_type, relationship_key_value)
  if(self.class._relationships[relationship_type.to_sym]).belongs_to?
    super
  end
end

def replace_polymorphic_to_one_link(relationship_type, relationship_key_value, relationship_key_type)
  if(self.class._relationships[relationship_type.to_sym]).belongs_to?
    super
  end
end

Start by reproducing the save failure for a has_one relationship using foreign_key_on: :related, then trace the relationship save path through replace_to_one_link and replace_polymorphic_to_one_link. Determine the intended behavior for related-item relationships and add or update coverage for the failing case; done means saving no longer calls an unavailable foreign_key= method and the intended relationship behavior is verified.