def set_relationship(key, value)
      existing = relationship_data[key]
      new_value = super
      set_dirty!(key, existing, new_value)
    end
def set_relationship(key, value)
      existing = relationship_data[key]
      new_value = super
      set_dirty!(key, existing, new_value)
    end
Overwrites {Relatable#set_relationship} to set the dirty state of the
 relationship. See {Dirty#set_dirty!} as well.