Random record in ActiveRecord

Question

I'm in need of getting a random record from a table via ActiveRecord. I've followed the example from Jamis Buck from 2006.

However, I've also come across another way via a Google search (can't attribute with a link due to new user restrictions):

 rand_id = rand(Model.count)
 rand_record = Model.first(:conditions => ["id >= ?", rand_id])

I'm curious how others on here have done it or if anyone knows what way would be more efficient.

Answer 1

I haven't found an ideal way to do this without at least two queries.

The following uses a randomly generated number (up to the current record count) as an offset.

offset = rand(Model.count)

# Rails 4
rand_record = Model.offset(offset).first

# Rails 3
rand_record = Model.first(:offset => offset)

To be honest, I've just been using ORDER BY RAND() or RANDOM() (depending on the database). It's not a performance issue if you don't have a performance issue.

Answer 2

In Rails 4 and 5, using Postgresql or SQLite, using RANDOM():

Model.order('RANDOM()').first

Presumably the same would work for MySQL with RAND()

Model.order('RAND()').first

This is about 2.5 times faster than the approach in the accepted answer.

Caveat: This is slow for large datasets with millions of records, so you might want to add a limit clause.

Answer 3

Your example code will start to behave inaccurately once records are deleted (it will unfairly favor items with lower ids)

You're probably better off using the random methods within your database. These vary depending on which DB you're using, but :order => "RAND()" works for mysql and :order => "RANDOM()" works for postgres

Model.first(:order => "RANDOM()") # postgres example

Answer 4

Benchmarking these two methods on MySQL 5.1.49, Ruby 1.9.2p180 on a products table with +5million records:

def random1
  rand_id = rand(Product.count)
  rand_record = Product.first(:conditions => [ "id >= ?", rand_id])
end

def random2
  if (c = Product.count) != 0
    Product.find(:first, :offset =>rand(c))
  end
end

n = 10
Benchmark.bm(7) do |x|
  x.report("next id:") { n.times {|i| random1 } }
  x.report("offset:")  { n.times {|i| random2 } }
end


             user     system      total        real
next id:  0.040000   0.000000   0.040000 (  0.225149)
offset :  0.020000   0.000000   0.020000 ( 35.234383)

Offset in MySQL appears to be much slower.

EDIT I also tried

Product.first(:order => "RAND()")

But I had to kill it after ~60 seconds. MySQL was "Copying to tmp table on disk". That's not going to work.

Answer 5

It doesn't have to be that hard.

ids = Model.pluck(:id)
random_model = Model.find(ids.sample)

pluck returns an array of all the id's in the table. The sample method on the array, returns a random id from the array.

This should perform well, with equal probability of selection and support for tables with deleted rows. You can even mix it with constraints.

User.where(favorite_day: "Friday").pluck(:id)

And thereby pick a random user who likes fridays rather than just any user.

Answer 6

I made a rails 3 gem to handle this:

https://github.com/spilliton/randumb

It allows you do do stuff like this:

Model.where(:column => "value").random(10)

Answer 7

It is not advised that that you use this solution, but if for some reason you really want to randomly select a record while only making one database query, you could use the sample method from the Ruby Array class, which allows you to select a random item from an array.

Model.all.sample

This method requires only database query, but it significantly slower than alternatives like Model.offset(rand(Model.count)).first which require two database queries, though the latter is still preferred.

Answer 8

I use this so often from the console I extend ActiveRecord in an initializer - Rails 4 example:

class ActiveRecord::Base
  def self.random
    self.limit(1).offset(rand(self.count)).first
  end
end

I can then call Foo.random to bring back a random record.

Answer 9

One query in Postgres:

User.order('RANDOM()').limit(3).to_sql # Postgres example
=> "SELECT "users".* FROM "users" ORDER BY RANDOM() LIMIT 3"

Using an offset, two queries:

offset = rand(User.count) # returns an integer between 0 and (User.count - 1)
Model.offset(offset).limit(1)

Answer 10

Reading all of these did not give me a lot of confidence about which of these would work best in my particular situation with Rails 5 and MySQL/Maria 5.5. So I tested some of the answers on ~ 65000 records, and have two take aways:

1. RAND() with a limit is a clear winner.
2. Do not use pluck + sample.

def random1
  Model.find(rand((Model.last.id + 1)))
end

def random2
  Model.order("RAND()").limit(1)
end

def random3
  Model.pluck(:id).sample
end

n = 100
Benchmark.bm(7) do |x|
  x.report("find:")    { n.times {|i| random1 } }
  x.report("order:")   { n.times {|i| random2 } }
  x.report("pluck:")   { n.times {|i| random3 } }
end

              user     system      total        real
find:     0.090000   0.000000   0.090000 (  0.127585)
order:    0.000000   0.000000   0.000000 (  0.002095)
pluck:    6.150000   0.000000   6.150000 (  8.292074)

This answer synthesizes, validates and updates Mohamed's answer, as well as Nami WANG's comment on the same and Florian Pilz's comment on the accepted answer - please send up votes to them!

Answer 11

You can use the Array method sample, the method sample returns a random object from an array, in order to use it you just need to exec in a simple ActiveRecord query that return a collection, for example:

User.all.sample

will return something like this:

#<User id: 25, name: "John Doe", email: "admin@example.info", created_at: "2018-04-16 19:31:12", updated_at: "2018-04-16 19:31:12">

Answer 12

If you need to select some random results within specified scope:

scope :male_names, -> { where(sex: 'm') }
number_of_results = 10

rand = Names.male_names.pluck(:id).sample(number_of_results)
Names.where(id: rand)

Answer 13

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

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

1. quick_random_records only cost 4.6ms totally.

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

3. the accepted answer 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.

Answer 14

The Ruby method for randomly picking an item from a list is sample. Wanting to create an efficient sample for ActiveRecord, and based on the previous answers, I used:

module ActiveRecord
  class Base
    def self.sample
      offset(rand(size)).first
    end
  end
end

I put this in lib/ext/sample.rb and then load it with this in config/initializers/monkey_patches.rb:

Dir[Rails.root.join('lib/ext/*.rb')].each { |file| require file }

This will be one query if the size of the model is already cached and two otherwise.

Answer 15

Rails 4.2 and Oracle:

For oracle you can set a scope on your Model like so:

scope :random_order, -> {order('DBMS_RANDOM.RANDOM')}

or

scope :random_order, -> {order('DBMS_RANDOM.VALUE')}

And then for a sample call it like this:

Model.random_order.take(10)

or

Model.random_order.limit(5)

of course you could also place an order without a scope like so:

Model.all.order('DBMS_RANDOM.RANDOM') # or DBMS_RANDOM.VALUE respectively

Answer 16

For MySQL database try: Model.order("RAND()").first

Answer 17

If you're using PostgreSQL 9.5+, you can take advantage of TABLESAMPLE to select a random record.

The two default sampling methods (SYSTEM and BERNOULLI) require that you specify the number of rows to return as a percentage of the total number of rows in the table.

-- Fetch 10% of the rows in the customers table.
SELECT * FROM customers TABLESAMPLE BERNOULLI(10);

This requires knowing the amount of records in the table to select the appropriate percentage, which may not be easy to find quickly. Fortunately, there is the tsm_system_rows module that allows you to specify the number of rows to return directly.

CREATE EXTENSION tsm_system_rows;

-- Fetch a single row from the customers table.
SELECT * FROM customers TABLESAMPLE SYSTEM_ROWS(1);

To use this within ActiveRecord, first enable the extension within a migration:

class EnableTsmSystemRowsExtension < ActiveRecord::Migration[5.0]
  def change
    enable_extension "tsm_system_rows"
  end
end

Then modify the from clause of the query:

customer = Customer.from("customers TABLESAMPLE SYSTEM_ROWS(1)").first

I don't know if the SYSTEM_ROWS sampling method will be entirely random or if it just returns the first row from a random page.

Most of this information was taken from a 2ndQuadrant blog post written by Gulcin Yildirim.

Answer 18

After seeing so many answers I decided to benchmark them all on my PostgreSQL(9.6.3) database. I use a smaller 100,000 table and got rid of the Model.order("RANDOM()").first since it was already two orders of magnitude slower.

Using a table with 2,500,000 entries with 10 columns the hands down winner was the pluck method being almost 8 times faster than the runner up(offset. I only ran this on a local server so that number might be inflated but its bigger enough that the pluck method is what I'll end up using. It's also worth noting that this might cause issues is you pluck more than 1 result at a time since each one of those will be unique aka less random.

Pluck wins running 100 time on my 25,000,000 row table Edit: actually this time includes the pluck in the loop if I take it out it it runs about as fast as simple iteration on the id. However; it does take up a fair amount of RAM.

RandomModel                 user     system      total        real
Model.find_by(id: i)       0.050000   0.010000   0.060000 (  0.059878)
Model.offset(rand(offset)) 0.030000   0.000000   0.030000 ( 55.282410)
Model.find(ids.sample)     6.450000   0.050000   6.500000 (  7.902458)

Here is the data running 2000 times on my 100,000 row table to rule out random

RandomModel       user     system      total        real
find_by:iterate  0.010000   0.000000   0.010000 (  0.006973)
offset           0.000000   0.000000   0.000000 (  0.132614)
"RANDOM()"       0.000000   0.000000   0.000000 ( 24.645371)
pluck            0.110000   0.020000   0.130000 (  0.175932)

Answer 19

I'm brand new to RoR but I got this to work for me:

 def random
    @cards = Card.all.sort_by { rand }
 end

It came from:

How to randomly sort (scramble) an array in Ruby?

Answer 20

What about to do:

rand_record = Model.find(Model.pluck(:id).sample)

For me is much clear

Answer 21

I try this of Sam's example on my App using rails 4.2.8 of Benchmark( I put 1..Category.count for random, because if the random takes a 0 it will produce an error(ActiveRecord::RecordNotFound: Couldn't find Category with 'id'=0)) and the mine was:

 def random1
2.4.1 :071?>   Category.find(rand(1..Category.count))
2.4.1 :072?>   end
 => :random1
2.4.1 :073 > def random2
2.4.1 :074?>    Category.offset(rand(1..Category.count))
2.4.1 :075?>   end
 => :random2
2.4.1 :076 > def random3
2.4.1 :077?>   Category.offset(rand(1..Category.count)).limit(rand(1..3))
2.4.1 :078?>   end
 => :random3
2.4.1 :079 > def random4
2.4.1 :080?>    Category.pluck(rand(1..Category.count))
2.4.1 :081?>
2.4.1 :082 >     end
 => :random4
2.4.1 :083 > n = 100
 => 100
2.4.1 :084 > Benchmark.bm(7) do |x|
2.4.1 :085 >     x.report("find") { n.times {|i| random1 } }
2.4.1 :086?>   x.report("offset") { n.times {|i| random2 } }
2.4.1 :087?>   x.report("offset_limit") { n.times {|i| random3 } }
2.4.1 :088?>   x.report("pluck") { n.times {|i| random4 } }
2.4.1 :089?>   end

                  user      system      total     real
find            0.070000   0.010000   0.080000 (0.118553)
offset          0.040000   0.010000   0.050000 (0.059276)
offset_limit    0.050000   0.000000   0.050000 (0.060849)
pluck           0.070000   0.020000   0.090000 (0.099065)

Answer 22

.order('RANDOM()').limit(limit) looks neat but is slow for large tables because it needs to fetch and sort all rows even if limit is 1 (internally in database but not in Rails). I'm not sure about MySQL but this happens in Postgres. More explanation in here and here.

One solution for large tables is .from("products TABLESAMPLE SYSTEM(0.5)") where 0.5 means 0.5%. However, I find this solution is still slow if you have WHERE conditions that filter out a lot of rows. I guess it's because TABLESAMPLE SYSTEM(0.5) fetch all rows before WHERE conditions apply.

Another solution for large tables (but not very random) is:

products_scope.limit(sample_size).sample(limit)

where sample_size can be 100 (but not too large otherwise it's slow and consumes a lot of memory), and limit can be 1. Note that although this is fast but it's not really random, it's random within sample_size records only.

PS: Benchmark results in answers above are not reliable (at least in Postgres) because some DB queries running at 2nd time can be significantly faster than running at 1st time, thanks to DB cache. And unfortunately there is no easy way to disable cache in Postgres to make these benchmarks reliable.

Answer 23

Very old question but with :

rand_record = Model.all.shuffle

You got an Array of record, sort by random order. No need gems or scripts.

If you want one record :

rand_record = Model.all.shuffle.first

Answer 24

Along with using RANDOM(), you can also throw this into a scope:

class Thing
  scope :random, -> (limit = 1) {
    order('RANDOM()').
    limit(limit)
  }
end

Or, if you don't fancy that as a scope, just throw it into a class method. Now Thing.random works along with Thing.random(n).