istan istan - 6 months ago 7
Ruby Question

Rails query through association limited to most recent record?

class User
has_many :books

I need a query that returns:

Users whose most recent book has :complete => true. i.e. If a user's most recent book has :complete => false, I do not want them in my result.

What I have so far

User.joins(:books).merge(Book.where(:complete => true))

which is a promising start but does not give me the result I need. I've tried adding an
.order("created_on desc").limit(1)

to the end of the above query but then I end up with only one result when I am expecting many.



If you aren't going to go with @rubyprince's ruby solution, this is actually a more complex DB query than ActiveRecord can handle in it's simplest form because it requires a sub-query. Here's how I would do this entirely with a query:

SELECT   users.*
FROM     users
         INNER JOIN books on books.user_id =
WHERE    books.created_on = ( SELECT  MAX(books.created_on)
                              FROM    books
                              WHERE   books.user_id =
         AND books.complete = true

To convert this into ActiveRecord I would do the following:

class User
  scope :last_book_completed, joins(:books)
    .where('books.created_on = (SELECT MAX(books.created_on) FROM books WHERE books.user_id =')
    .where('books.complete = true')

You can then get a list of all users that have a last completed book by doing the following: