Finding objects that are not associated in has_many_through - ruby-on-rails

Question

I have simple classes like these:
class Book
has_many :book_categorizations
has_many :categories, through: :book_categorizations, source: :book_category
end
class BookCategorizations
belongs_to :book
belongs_to :book_category
end
class BookCategory
has_many :book_categorizations
has_many :books, through: :book_categorizations
end
I would like to find Books that have no category. How can I query that using where?

Answer 1

You could add scope with an LEFT JOIN to your model:
# in book.rb
scope :without_categories, lambda {
joins('LEFT JOIN book_categorizations ON books.id = book_categorizations.book_id').
where(book_categorizations: { book_category_id: nil })
}
Which could be used like:
Book.without_categories
#=> returns books without a category
How it works:
Imaging you have a fruits and a colors table:
fruits
id | name
1 | Apple
2 | Orange
3 | Banana
colors
id | name
1 | black
2 | red
3 | yellow
And a colors_fruits join table:
colors_fruits
color_id | fruit_id
2 | 1 # red Apple
3 | 3 # yellow Banana
Since Rails' joins method generates INNER JOIN, all joins would only return fruits that have at least one color. The orange wouldn't be in the list, because it does not have a color (therefore no join is possible):
Fruit.joins(:colors)
#=> red Apple, yellow Banana (simplified)
But when we are interested into fruits that do not have an color, then we need an LEFT JOIN. A LEFT JOIN includes all elements from the left table - even if there is not matching on the right table (unfortunately there is no Rails helper for this kind of joins):
Fruits.joins('LEFT JOIN colors_fruits ON colors_fruits.fruits_id = fruits.id')
This generates a result like:
id | color | fruit_id | color_id
1 | Apple | NULL | NULL
2 | Orange | 2 | 1
3 | Banana | 3 | 3
Now we just need to exclude the ones that do not have a color_id
Fruits.joins('LEFT JOIN colors_fruits ON colors_fruits.fruits_id = fruits.id').
where(colors_fruits: { color_id: nil })
You might want to read about the different types of SQL JOINS. And there is this well known diagram about joins.