如何使用 Rails 优化此查询

Question

I have an array of book ids and I need to iterate each and save the number of comments for each in a Hash (@book_comments) that will have the book_id as the key and the number of comments for that book as a value. Book has many comments, comment belong to books.

@book_ids.map {|id| @book_comments[id] = Book.find(id).comments.size}

This will hit the DB with these two queries for the amount of ids I have on my array.

SELECT `books`.* FROM `books` WHERE `books`.`id` = ? LIMIT 1
SELECT COUNT(*) FROM `comments` WHERE `comments`.`book_id` = ?

Surely there's a better way. If you know, please teach me. Thank you.

Answer 1

这应该得到你的哈希：

@book_comments = Comment.where(book_id: @book_ids).group(:book_id).count

Answer 2

Use this code:

comment_counts_by_book = Book.select('books.id, count(comments.id) as comments_count').
    joins('left outer join comments on comments.book_id = books.id').
    group('books.id').inject({}) { |h, book| h[book.id] = book.comments_count; h }

comment_counts_by_book
# => {1=>2, 2=>5, ...}

It does a single database query returning Book objects with only :id and :comments_count attributes filled out. The result is then transformed using inject to a hash with book ids as keys and comments counts as values.

Answer 3

If you're using Rails 4 you can do the following as well:

Comment.eager_load(:book).group(:book).count

It'll generate something like this:

=> #{1=>2, 2=>4, 3=>9}

Answer 4

Does this work for you? I'm not sure if I understood the question correctly. Let me know.

Book.find(@book_ids).map(&:comments).size

Update

Is this closer to what you want?

@book_ids.map do |id|
  {
    id => Book.find(id).map(&:comments).size
  }
end

Try this, too

{}.tap do |hash|
  @book_ids.each do |book_id|
    hash[book_id] = Book.find(book_id).map(&:comments).size
  end
end

Answer 5

You should use includes to eager load the comments then iterate over that query.

books = Book.where(id: @book_ids).includes(:comments)

books.each do |book|
  @book_comments[book.id] = book.comments.length
end