使用连接选择属性并在导轨中选择

Question

我有一个 TopicRelation 类，它使用 source_topic_id 和 target_topic_id 在两个主题之间建立关系。这是模型：

class TopicRelation < ActiveRecord::Base
  belongs_to :source_topic, class_name: 'Topic'
  belongs_to :target_topic, class_name: 'Topic'
end

我正在尝试仅提取源主题的名称：

TopicRelation.joins(:source_topic, :target_topic)
  .where('source_topic_id=?', topic.id)
  .select('source_topics.name')

但是，这给了我以下错误：

TopicRelation Load (113.7ms)  SELECT  source_topics.name FROM `topic_relations` INNER JOIN `topics` ON `topics`.`id` = `topic_relations`.`source_topic_id` INNER JOIN `topics` `target_topics_topic_relations` ON `target_topics_topic_relations`.`id` = `topic_relations`.`target_topic_id` WHERE (source_topic_id=1) LIMIT 10
Mysql2::Error: Unknown column 'source_topics.name' in 'field list': SELECT  source_topics.name FROM `topic_relations` INNER JOIN `topics` ON `topics`.`id` = `topic_relations`.`source_topic_id` INNER JOIN `topics` `target_topics_topic_relations` ON `target_topics_topic_relations`.`id` = `topic_relations`.`target_topic_id` WHERE (source_topic_id=1) LIMIT 10
ActiveRecord::StatementInvalid: Mysql2::Error: Unknown column 'source_topics.name' in 'field list': SELECT  source_topics.name FROM `topic_relations` INNER JOIN `topics` ON `topics`.`id` = `topic_relations`.`source_topic_id` INNER JOIN `topics` `target_topics_topic_relations` ON `target_topics_topic_relations`.`id` = `topic_relations`.`target_topic_id` WHERE (source_topic_id=1) LIMIT 10

是否可以使用这种方法同时为source_topic 和target_topic 选择名称？

Answer 1

select 使用 db 表的名称，在您的情况下可能是 topics：

TopicRelation
  .joins('INNER JOIN topics target_topics ON topic_relations.target_topic_id = target_topics.id')
  .joins('INNER JOIN topics source_topics ON topic_relations.source_topic_id = source_topics.id')
  .where(source_topic: topic)
  .select('source_topics.name')

这也会返回TopicRelation的列表；如果只需要主题名称，请使用方法pluck 而不是select