ruby on rails how to get latest records from has many relationship

Question

I am new to Ruby on Rails. I have a problem- how to get only latest records from has many relationship in model. My code is like this:

class User < ActiveRecord::Base 
  has_many :books
end

class Book < ActiveRecord::Base
  belongs_to :user

  scope :rating, -> { where(rating: 5) }
end

What I would like to achieve is: I'd like to get all latest (last) book for each user and than filter this books using scope rating. I can do this using for each loop (Looping through User.books and get book.last)- but this is very inefficient because I have a lot of data. Do you have any ideas how I can do this? maybe some kind of scope chaining with each other?

Answer 1

You can do this if you want to avoid writing SQL:

class Book < ActiveRecord::Base
  scope :latest, -> do
    book_ids_hash = Book.group(:user_id).maximum(:id)
    book_ids = book_ids_hash.values
    where(id: book_ids)
  end
end

Then all you have to do is:

Book.rating.latest

Answer 2

Try this:

User.find_each do |user|
  puts 'User top 10 books:'

  user.books.rating.order('id DESC').limit(10).each do |book|
    puts book.title 
  end
end

I'd like to get all latest (last) book for each user

In the example above, note the .order('id DESC').limit(10) . This will sort books by ID in descending order.

Answer 3

每位用户一本书，然后将结果限制为评分为5的用户，一个查询无循环或任何问题。

Book.order(created_at: :desc).group(:user_id).rating

Answer 4

Just use last method and pass the number of records you want to the parameter :

user.books.last(5).rating

this will give you the 5 last books and apply the rating to it.

see http://api.rubyonrails.org/classes/ActiveRecord/FinderMethods.html#method-i-last