Rails AR + Mysql-排除电影流派无法正常工作

Question

Let's say I have a movie database with genres and I would like to show all movies excluding some specific genres. I have associations all set up and working just fine.

So to do this, i came across the gem named Squeel and wrote this piece of code

movies = Movie.joins(:movie_genres).approved

# This is just an array of id's
included_genres = params[:genres][:included_genres]
excluded_genres = params[:genres][:excluded_genres]

if included_genres

  # This works just fine
  movies = movies.where(movie_genres: {genre_id: included_genres})

  # This works too but IMO MySQL IN operator is faster.
  #movies = movies.where{movie_genres.genre_id.eq_any included_genres}

end

if excluded_genres

  # Here is where the problems occurs, it generates perfectly nice SQL 
  # but for some reason it doesn't work as expected.
  movies = movies.where{movie_genres.genre_id.not_eq_any excluded_genres}

  # If we do it this way, it will only not match those movies 
  # that have all the selected genres at once, i need to match 
  # all movies that have atleast one of the inputted genres
  #movies = movies.where("genre_id NOT IN (?)", excluded_genres)
  #movies = movies.where{movie_genres.genre_id.not_eq_all excluded_genres}    

end

movies = movies.group(:id)

So yeah, thanks to Squeel we get this SQL:

SELECT `movies`.* FROM `movies` 
INNER JOIN `movie_genres` ON `movie_genres`.`movie_id` = `movies`.`id` 
WHERE ((`movie_genres`.`genre_id` != 10 OR `movie_genres`.`genre_id` != 12)) 
GROUP BY `movies`.`id`;

That's without included genres as they work as expected.

So in result, the where condition looks like it doesn't even apply as i see no difference with or without it.

See screenshot:

Does anyone have any ideas why it doesn't works and where should i start digging?

All help is much appreciated.

Answer 1

The problem, is in logic, not sql. With "OR" logic you expect to get lines meeting any of conditions, so for your query you will get the line with id 10, because 10 != 12. Using WHERE IN - fine way

Answer 2

Solved this problem by using named scope

scope :excluding_genres, 
  lambda { |genres| { 
    :conditions => 
       ['movies.id NOT IN 
       (SELECT movies.id FROM movies 
        LEFT JOIN movie_genres ON movies.id = movie_genres.movie_id 
        WHERE movie_genres.genre_id IN (?))', genres] 
  }}