使用联接选择属性并在Rails中选择

Question

I have a TopicRelation class which builds a relationship between two topics, using source_topic_id and target_topic_id . Here is the model:

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

I am trying to pull just the name of the source topic:

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

However, this gives me the following error:

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

Is it possible to select the name for both source_topic and target_topic using this approach?

Answer 1

The select use the name of the db table, which is likely topics in your case :

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')

Also this will return a list of TopicRelation ; If you want only the name of the topics, use the method pluck instead of select