Rails查询通过仅限于最新记录的关联？

Question

class User 
has_many :books

I need a query that returns:

Users whose most recent book has :complete => true. ie 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.

Thanks!

Answer 1

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 = users.id
WHERE    books.created_on = ( SELECT  MAX(books.created_on)
                              FROM    books
                              WHERE   books.user_id = users.id)
         AND books.complete = true
GROUP BY users.id

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 = users.id)')
    .where('books.complete = true')
    .group('users.id')
end

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

User.last_book_completed

Answer 2

This adds a little overhead, but saves complexity and increases speed later when it matters.

Add a "most_recent" column to books. Make sure you add an index.

class AddMostRecentToBooks < ActiveRecord::Migration

  def self.change
    add_column :books, :most_recent, :boolean, :default => false, :null => false
  end
  add_index :books, :most_recent, where: :most_recent  # partial index

end

Then, when you save a book, update most_recent

class Book < ActiveRecord::Base

  on_save :mark_most_recent

  def mark_most_recent
    user.books.order(:created_at => :desc).offset(1).update_all(:most_recent => false)
    user.books.order(:created_at => :desc).limit(1).update_all(:most_recent => true)
  end

end

Now, for your query

class User < ActiveRecord::Base

  # Could also include and preload most-recent book this way for lists if you wanted
  has_one :most_recent_book, -> { where(:most_recent => true) }, :class_name => 'Book'

  scope :last_book_completed, -> { joins(:books).where(:books => { :most_recent => true, :complete => true })
end

This allows you to write it like this and the result is a Relation to be used with other scopes.

User.last_book_completed

Answer 3

我想不出一种方法可以在一个查询中执行它，但你可以这样做：

User.all.select { |user| user.books.order("created_at desc").limit(1).complete }

Answer 4

I recently came across a similar problem and here is how I solved it:

most_recent_book_ids = User.all.map {|user| user.books.last.id }
results = User.joins(:books).where('books.id in (?) AND books.complete == ?', most_recent_book_ids, true).uniq

This way we only use ActiveRecord methods (no extra SQL) and can reuse it when considering any subset of books for users (first, last, last n books, etc...). You need the last 'uniq' cause otherwise each user would appear twice..

Answer 5

based off Pan's answer but with a left join instead of an inner join, and no grouping:

scope :with_last_book_complete, -> {
  subquery = Book.select(:id)
                 .where("books.user_id = users.id")
                 .order(created_at: :desc).limit(1)

  joins(<<-SQL).where(books: { complete: true })
    LEFT JOIN books
      ON books.user_id = users.id
      AND books.id = (#{subquery.to_sql})
  SQL
}

Answer 6

you should be using scopes here - they will make your life much simple (this is a rails3 example)

In your book.rb model

scope :completed, where("complete = ?", true)
scope :recent, order("created_on ASC")

Then you can call User.books.recent.completed to get what you want