Rails query through association limited to most recent record?

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.idWHERE    books.created_on = ( SELECT  MAX(books.created_on)                              FROM    books                              WHERE   books.user_id = users.id)         AND books.complete = trueGROUP 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

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 indexend

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)  endend

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

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..