您可以分别定义主has_many关联和has_one关联，如下所示：

class ClinicalSystem
  has_many :tempalte_messages
  has_one :reschedule_template_message, -> { CONDITION_FOR_RESCHEDULE }, class_name: TemplateMessage.name
  has_one :cancellation_template_message, -> { CONDITION_FOR_CANCELLATION }, class_name: TemplateMessage.name
end

您可以使用foreign_key和class_name实现这一点

class ClinicalSystem
  has_many :tempalte_messages
  belongs_to :reschedule_template_message, foreign_key: 'reschedule_template_message_id', class_name: 'TemplateMessage'
  belongs_to :cancellation_template_message, foreign_key: 'cancellation_template_message_id', class_name: 'TemplateMessage'
end

您还需要添加迁移，以便在临床表中添加foreign_key reschedule_template_message_id和cancellation_template_message_id