如果包括且仅当查询不适用于Rails和Postgres时，才使用join进行所有操作

Question

What I want is to query for all media that only has "fra" language associated through the habtm association. Currently, this query returns Media instances that include "fra", so eg ["fra", "eng"]. I use Postgres.

So, (1) I want to find the media that has exactly the language 'x' and no other, (2) find the media that has exactly the languages 'x' and 'y', (3) find the media that has exactly the languages 'x', 'y' and 'z' etc.

class Media < ActiveRecord::Base
    has_and_belongs_to_many :audio_language_records, :join_table => "audio_languages_media", :class_name => "Language"
end

class Language < ActiveRecord::Base
  attr_accessor :code
end

I have tried this, but it returns all media that includes "fra", but not only "fra"

Media.joins(:audio_language_records).where("languages.code = ? AND languages.code != ?", "fra", "eng")

The following does not work as intended and returns the same result

Media.joins(:audio_language_records).where("languages.code IN (?) AND languages.code NOT IN (?)", ["fra"], ["eng"])

Answer 1

So you need to find the Media for which there exists a language.code of 'eng' but no entries in the join table for the same media_id but a different language_id.

SELECT * FROM media m1
  JOIN audio_languages_media alm1 ON alm1.media_id = m1.id
  JOIN languages l1 ON alm1.language_id = l1.id 
  WHERE NOT EXISTS(
    SELECT 1 FROM audio_languages_media alm2
    WHERE alm1.language_id != alm2.language_id
    AND alm1.media_id = alm2.media_id
  )
  AND l1.code = 'eng';

Let us know if this is the right db query so we can help with the AREL.

Edit: Query for when you want to find a media that is in at least 'eng' and 'fra'

SELECT * FROM media m1
  WHERE(
    SELECT count(*) FROM audio_languages_media alm2
    JOIN languages l2 ON alm2.language_id = l2.id
    WHERE l2.code in ('eng','fra')
    AND alm2.media_id = m1.id
  ) > 1;

Edit: Add @chinshr's query

If you want media that has only exactly 'eng' and 'fra'

SELECT * FROM media m1
  WHERE(
    SELECT count(*) FROM audio_languages_media alm2
    JOIN languages l2 ON alm2.language_id = l2.id
    WHERE l2.code IN ('eng','fra')
    AND alm2.media_id = m1.id
    AND (
      SELECT count(*) FROM audio_languages_media alm2 
      WHERE alm2.media_id = media.id
    ) = 2
  ) = 2;

This query can be tweaked for more or less languages by adding/removing from the IN array, and adjusting the count at the end to be equal to the number of elements in the IN array.

For this to work reliably, you must have a unique index on audio_languages_media(media_id, language_id);

Answer 2

You only have one value per row so every value that is "fra" can't be "eng" or any other thing. You're gonna have to check two rows. It can be done in SQL like this for example:

SELECT * 
FROM languages t1
LEFT JOIN languages t2
    ON t1.id=t2.id AND t2.code="eng"
WHERE t1.code="fra" AND t2.code IS NULL;