query: has_many where all the child fields are nil

问题

I'm using Rails 3.2 with ActiveRecord and MySQL and I have models with one to many association:

class Author
  has_many :books
end

class Book
  belongs_to :author
  attr_accessible :review
end

I want to find authors that have all the books without review. I tried:

Author.includes(:books).where('book.review IS NIL')

but is obviously didn't work, because it finds authors that have at least one book without review. What query should I use?

回答1:

SQL is quite simple:

SELECT authors.name, count(books.review is not null) 
FROM authors LEFT JOIN books ON (authors.id=books.author_id) 
GROUP BY authors.name
HAVING count(books.review) == 0

Translating it to the AR query language may take me some time... OK, so it seems to look like this:

Author.count('books.review', joins: :books, select: 'name', 
              group:'name', having: 'count_books_review=0')

As for me SQL looks much less weird then this ;-)

回答2:

Basing on the WRz answer I prepared my own query:

Author.joins(:books).group('authors.id').having("count(books.reviews)=0")

It's better suited for me, because it returns an AR Relation (and WRz's query returns a Hash).

回答3:

Try this

Author.joins(:books).where('books.review is null')

edit: This will fetch all the authors with at least one book with no review. I just realized your question is a bit different.

It would be something like this.

Authors.joins(:books).select('authors.*, count(books.id) as
total_books, count('books.review is null')
as books_without_review.group('authors.id').having(total_books ==
books_without_review)

P.S: This is not the exact syntax and it is untested

回答4:

Try the following code.

class Author
  has_many :books
end

class Book
  belongs_to :author
  attr_accessible :review
end

authors = Author.all.collect do |author|
            if author.books.where(:review => nil).size == author.books.size
              author
            end
          end

authors.compact!

After this code, authors will be an array containing all the authors having all the books unreviewed. Also note that I changed the author association in Book model to belongs_to instead of has_one. It is always a good practice to have has_many relation on one side and belongs_to association on the other side.

来源：https://stackoverflow.com/questions/15659503/query-has-many-where-all-the-child-fields-are-nil

标签

mysql

sql

ruby-on-rails

ruby

activerecord