def add_study(study)
      raise ArgumentError, "Invalid argument 'study'. Expected Study, got #{study.class}." unless study.is_a?(Study)
      # Do not add it again if the study already belongs to this instance:
      @studies << study unless @associated_studies[study.uid]
      @associated_studies[study.uid] = study
    end
def add_study(study)
      raise ArgumentError, "Invalid argument 'study'. Expected Study, got #{study.class}." unless study.is_a?(Study)
      # Do not add it again if the study already belongs to this instance:
      @studies << study unless @associated_studies[study.uid]
      @associated_studies[study.uid] = study
    end
Adds a Study to this Patient.

 @param [Study] study a study instance to be associated with this patient