Question

Say I have 2 models, Album and Image, and a join model named AlbumImage (and 3 corresponding tables in the database).

The models share a has-and-belongs-to-many association – Album has many images through album_images, and vice versa.

Tl;dr version

How can I find every image which does not appear within a given album?

Long version

I’m looking for something equivalent to the following SQL:

SELECT * FROM images
WHERE NOT EXISTS (
    SELECT * FROM album_images
    WHERE album_images.image_id = images.id
    AND album_images.album_id = ?
);

i.e. select every image where there does not exist a row in the album_images table with the same image ID and the ID of a given album.

But I unfortunately have no idea how to express this in Rails’ query syntax.

Answer 1

Try this:

Option 1

Image.all(:conditions => ["images.id NOT IN ( 
  SELECT a.image_id FROM album_images a WHERE a.album_id = ?)", alb_id])

This approach is better than using NOT EXISTS as sub query result here is cached by DB.

Option 2

Using LEFT OUTER JOIN.

Image.all(:joins => "LEFT OUTER JOIN album_images a 
    ON a.album_id = #{alb_id} AND a.image_id = images.id",
 :conditions => "a.image_id IS NULL")

Option 3

If the number of images per album is limited and you dont mind incurring the cost of one extra query:

conditions = ["id NOT IN (?)", 
  @album.album_images.map(&:image_id)] unless @album.album_images.empty?

Image.all(:conditions => conditions)