为什么我的查询没有提供单一的结果？

Question

I have following query with group by and having plus count . It should return only those rows having columns specified in the group by clause with the count specified by having count x .

I think the following query should be correct:

@articles = Article.joins(:taggings)
                   .where(taggings: { tag_id: @article_tags_ids, taggable_type: "Article"})
                   .group('articles.id')
                   .having("count(taggings.tag_id) = ?", @article_tags_count)

In this case, it should only return those articles, that have those and exactly those tags the user searched for and none more.

Schema is from this tutorial: https://cobwwweb.com/rails-has-many-through-polymorphic-association

It creates this SQL

SELECT
  `articles`.* 
FROM
  `articles` 
INNER JOIN `taggings` 
  ON `taggings`.`taggable_id` = `articles`.`id` 
  AND `taggings`.`taggable_type` = 'Article' 
WHERE
  `taggings`.`tag_id` IN (1, 3) 
  AND `taggings`.`taggable_type` = 'Article' 
GROUP BY
  articles.id 
HAVING
  (count(taggings.tag_id) = 2) 
LIMIT 11

Answer 1

The solution is following SQL Query with not one but two joins on the join-table. I don't know what substantial difference that makes, but obviously for the database it does make one, even if it can match articles with tags with a single join already.

SELECT  
`articles`.*
FROM `articles`
JOIN `taggings` 
ON `articles`.`id` = `taggings`.`taggable_id` 
JOIN `tags` ON `taggings`.`taggable_id` = `tags`.id 
WHERE `tags`.`id` in (1,2,3) 
GROUP BY `articles`.id 
HAVING( COUNT(`tags`.`id`) = 3);

Such that when you have two articles, article-one and article-two, where article-one matches tag1, tag2, tag3 and article-two also matches tag1, tag2, tag3 and over that tag4, the query only returns article-one. The conjunction is a "this" and "nothing more" and not an or

Yours

vonSpotz