Ruby中的唯一随机记录

Question

I have a Question table in which there is id# and title as columns. Now, I need to randomly select 5 questions from the table. I am seeing people are using:

Question.order("RANDOM()").limit(5) //using postgre

Till now I have:

def selectr
        @randquestion=[]
        while @randquestion.length<3 do
            Question.uncached do
                ques=Question.order("RANDOM()").first
                @randquestion << ques
            end
        end
end

I found uncaching from Ruby on Rails Active Record RANDOM() always the same within a loop .

But I am not sure if this will give me unique questions. I want 3 unique questions only.

Answer 1

You can do this by fetching each question via a separate SQL request, with subsequent requests excluding IDs for records already seen. Here's an example, tested with Rails 5.2 and MySQL:

def random_questions(number)
  already_seen = []
  number.times.map do
    question = Question.order('RAND()').where.not(id: already_seen).first
    already_seen << question.id
    question
  end
end

If you try this out with random_questions(3) , you'll see:

Question Load (1.5ms)  SELECT  `questions`.* FROM `questions` WHERE 1=1 ORDER BY RAND() LIMIT 1
Question Load (1.4ms)  SELECT  `questions`.* FROM `questions` WHERE `questions`.`id` != 2 ORDER BY RAND() LIMIT 1
Question Load (1.3ms)  SELECT  `questions`.* FROM `questions` WHERE `questions`.`id` NOT IN (2, 1) ORDER BY RAND() LIMIT 1

As an aside, please note that order('RAND()') triggers a deprecation warning in newer versions of Rails:

DEPRECATION WARNING: Dangerous query method (method whose arguments are used as raw SQL) called with non-attribute argument(s): "RAND()". Non-attribute arguments will be disallowed in Rails 6.0. This method should not be called with user-provided values, such as request parameters or model attributes. Known-safe values can be passed by wrapping them in Arel.sql().

To avoid this warning, use .order(Arel.sql('RAND()')) instead.