Rails选择随机记录

Question

I don't know if I'm just looking in the wrong places here or what, but does active record have a method for retrieving a random object?

Something like?

@user = User.random

Or... well since that method doesn't exist is there some amazing "Rails Way" of doing this, I always seem to be to verbose. I'm using mysql as well.

Answer 1

Most of the examples I've seen that do this end up counting the rows in the table, then generating a random number to choose one. This is because alternatives such as RAND() are inefficient in that they actually get every row and assign them a random number, or so I've read (and are database specific I think).

You can add a method like the one I found here .

module ActiveRecord
  class Base
    def self.random
      if (c = count) != 0
        find(:first, :offset =>rand(c))
      end
    end
  end
end

This will make it so any Model you use has a method called random which works in the way I described above: generates a random number within the count of the rows in the table, then fetches the row associated with that random number. So basically, you're only doing one fetch which is what you probably prefer :)

You can also take a look at this rails plugin .

Answer 2

We found that offsets ran very slowly on MySql for a large table. Instead of using offset like:

model.find(:first, :offset =>rand(c))

...we found the following technique ran more than 10x faster (fixed off by 1):

max_id = Model.maximum("id")
min_id = Model.minimum("id")
id_range = max_id - min_id + 1
random_id = min_id + rand(id_range).to_i
Model.find(:first, :conditions => "id >= #{random_id}", :limit => 1, :order => "id")

Answer 3

尝试使用Array的示例方法：

@user = User.all.sample(1)

Answer 4

In Rails 4 I would extend ActiveRecord::Relation :

class ActiveRecord::Relation
  def random
    offset(rand(count))
  end
end

This way you can use scopes:

SomeModel.all.random.first # Return one random record
SomeModel.some_scope.another_scope.random.first

Answer 5

I'd use a named scope. Just throw this into your User model.

named_scope :random, :order=>'RAND()', :limit=>1

The random function isn't the same in each database though. SQLite and others use RANDOM() but you'll need to use RAND() for MySQL.

If you'd like to be able to grab more than one random row you can try this.

named_scope :random, lambda { |*args| { :order=>'RAND()', :limit=>args[0] || 1 } }

If you call User.random it will default to 1 but you can also call User.random(3) if you want more than one.

Answer 6

If you would need a random record but only within certain criteria you could use "random_where" from this code:

module ActiveRecord
  class Base
    def self.random
      if (c = count) != 0
        find(:first, :offset =>rand(c))
      end
    end

    def self.random_where(*params)
      if (c = where(*params).count) != 0
        where(*params).find(:first, :offset =>rand(c))
      end
    end

  end
end

For eg :

@user = User.random_where("active = 1")

This function is very useful for displaying random products based on some additional criteria

Answer 7

Here is the best solution for getting random records from database. RoR provide everything in ease of use.

For getting random records from DB use sample , below is the description for that with example.

Backport of Array#sample based on Marc-Andre Lafortune's github.com/marcandre/backports/ Returns a random element or n random elements from the array. If the array is empty and n is nil, returns nil. If n is passed and its value is less than 0, it raises an ArgumentError exception. If the value of n is equal or greater than 0 it returns [].

[1,2,3,4,5,6].sample     # => 4     
[1,2,3,4,5,6].sample(3)  # => [2, 4, 5]     
[1,2,3,4,5,6].sample(-3) # => ArgumentError: negative array size     
[].sample     # => nil     
[].sample(3)  # => []     

You can use condition with as per your requirement like below example.

User.where(active: true).sample(5)

it will return randomly 5 active user's from User table

For more help please visit : http://apidock.com/rails/Array/sample

Answer 8

Strongly Recommend this gem for random records, which is specially designed for table with lots of data rows:

https://github.com/haopingfan/quick_random_records

Simple Usage:

@user = User.random_records(1).take

---

All other answers perform badly with large database, except this gem:

1. quick_random_records only cost 4.6ms totally.

2. the accepted answer User.order('RAND()').limit(10) cost 733.0ms .

3. the offset approach cost 245.4ms totally.

4. the User.all.sample(10) approach cost 573.4ms .

Note: My table only has 120,000 users. The more records you have, the more enormous the difference of performance will be.

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UPDATE:

Perform on table with 550,000 rows

1. Model.where(id: Model.pluck(:id).sample(10)) cost 1384.0ms

2. gem: quick_random_records only cost 6.4ms totally