Ruby on Rails - Has Many, Through: find multiple conditions - ruby-on-rails

Question

I realised its quite difficult to explain my problem with only words, so i'm going to use an example to describe what i am trying to do instead.
So for example:
#model Book
has_many: book_genres
has_many: genres, through: :book_genres
#model Genre
has_many: book_genres
has_many: books, through: :book_genres
So finding books that belong to one genre only would be relatively straightforward, such as:
#method in books model
def self.find_books(genre)
#g = Genre.where('name LIKE ?' , "#{genre}").take
#b = #g.books
#get all the books that are of that genre
end
So in rails console i can do Book.find_books("Fiction") and then i would get all the books that are of fiction genre.
But how can i find all the books that are both "Young Adult" and "Fiction" ? Or what if i would like to query for books that have 3 genres, such as "Young Adult", "Fiction" and "Romance" ?
I could do g = Genre.where(name: ["Young Adult", "Fiction", "Romance"]) but subsequent to that i cannot do g.books and get all the books that are related to this 3 genres.
I am actually quite bad with active record so im not even sure if theres a better way to query through Books directly instead of finding Genre then finding all books that are associated with it.
But what i cannot wrap my head around is how do i get all the books that have multiple (specific)genres?
UPDATE:
So the current answers provided Book.joins(:genres).where("genres.name" => ["Young Adult", "Fiction", "Romance"]) works, but the problem is it returns all books that has the genre of Young Adult OR Fiction OR Romance.
What query do i pass so that the books return has ALL 3 Genres and not only 1 or 2 out of the 3?

Answer 1

Matching any of the given genres
The following should work for both an Array and a String:
Book.joins(:genres).where("genres.name" => ["Young Adult", "Fiction", "Romance"])
Book.joins(:genres).where("genres.name" => "Young Adult")
In general, it's better to pass a Hash to where, rather than trying to write a SQL snippet yourself.
See the Rails Guides for more details:
http://guides.rubyonrails.org/active_record_querying.html#hash-conditions
http://guides.rubyonrails.org/active_record_querying.html#specifying-conditions-on-the-joined-tables
Matching all of the given genres with one query
A single query could be built and then passed to .find_by_query:
def self.in_genres(genres)
sql = genres.
map { |name| Book.joins(:genres).where("genres.name" => name) }.
map { |relation| "(#{relation.to_sql})" }.
join(" INTERSECT ")
find_by_sql(sql)
end
This means that calling Book.in_genres(["Young Adult", "Fiction", "Romance"]) will run a query that looks something like this:
(SELECT books.* FROM books INNER JOIN … WHERE genres.name = 'Young Adult')
INTERSECT
(SELECT books.* FROM books INNER JOIN … WHERE genres.name = 'Fiction')
INTERSECT
(SELECT books.* FROM books INNER JOIN … WHERE genres.name = 'Romance');
It has the upside of letting the database do the heavy lifting of combining the result sets.
The downside is that we're using raw SQL, so we can't chain this with other ActiveRecord methods, for example Books.order(:title).in_genres(["Young Adult", "Fiction"]) will ignore the ORDER BY clause we've tried to add.
We're also manipulating SQL queries as strings. It's possible we could avoid this using Arel, but the way Rails and Arel handle binding query values makes this pretty complicated.
Matching all of the given genres with multiple query
It's also possible to use multiple queries:
def self.in_genres(genres)
ids = genres.
map { |name| Book.joins(:genres).where("genres.name" => name) }.
map { |relation| relation.pluck(:id).to_set }.
inject(:intersection).to_a
where(id: ids)
end
This means that calling Book.in_genres(["Young Adult", "Fiction", "Romance"]) will run four queries that look something like this:
SELECT id FROM books INNER JOIN … WHERE genres.name = 'Young Adult';
SELECT id FROM books INNER JOIN … WHERE genres.name = 'Fiction';
SELECT id FROM books INNER JOIN … WHERE genres.name = 'Romance';
SELECT * FROM books WHERE id IN (1, 3, …);
The downside here is that for N genres, we're making N+1 queries. The upside is that this can be combined with other ActiveRecord methods; Books.order(:title).in_genres(["Young Adult", "Fiction"]) will do our genre filtering, and sort by title.

Answer 2

I didn't try this but I think it will work
Book.joins(:genres).where("genres.name IN (?)", ["Young Adult", "Fiction", "Romance"])

Answer 3

Here is how I would do it in SQL:
SELECT *
FROM books
WHERE id IN (
SELECT bg.book_id
FROM book_genres bg
INNER JOIN genres g
ON g.id = bg.genre_id
WHERE g.name LIKE 'Young Adult'
INTERSECT
SELECT bg.book_id
FROM book_genres bg
INNER JOIN genres g
ON g.id = bg.genre_id
WHERE g.name LIKE 'Fiction'
INTERSECT
...
)
The inner query will contain only books belonging to all the genres you ask about.
Here is how I'd do it in ActiveRecord:
# book.rb
def self.in_genres(genre_names)
subquery = genre_names.map{|n|
<<-EOQ
SELECT bg.book_id
FROM book_genres bg
INNER JOIN genres g
ON g.id = bg.genre_id
WHERE g.name LIKE ?
EOQ
}.join("\nINTERSECT\n")
where(<<-EOQ, *genre_names)
id IN (
#{subquery}
)
EOQ
end
Note that I am using ? to avoid sql injection vulnerabilities, which is a problem in the code you proposed in your question.
Another approach would be to use multiple EXISTS conditions with correlated sub-queries:
SELECT *
FROM books
WHERE EXISTS (SELECT 1
FROM book_genres bg
INNER JOIN genres g
ON g.id = bg.genre_id
WHERE g.name LIKE 'Young Adult'
AND bg.book_id = books.id)
AND EXISTS (SELECT 1
FROM book_genres bg
INNER JOIN genres g
ON g.id = bg.genre_id
WHERE g.name LIKE 'Fiction'
AND bg.book_id = books.id)
AND ...
You'd construct this query in ActiveRecord similarly to the first approach. I'm not sure which would be faster, so you could try both if you like.
Here is yet another way to do the SQL---possibly fastest:
SELECT *
FROM books
WHERE id IN (
SELECT bg.book_id
FROM book_genres bg
INNER JOIN genres g
ON g.id = bg.genre_id
WHERE (g.name LIKE 'Young Adult' OR g.name LIKE 'Fiction' OR ...)
GROUP BY bg.book_id
HAVING COUNT(DISTINCT bg.genre_id) >= 2 -- or 3, or whatever
)