Unique Random Numbers & Random Rows from Large ActiveRecord Tables

I was looking for an easy way to get unique and random numbers. In my case, I need a small quantity of numbers from a large pool. Here is the best solution I could come up with (derived from a message board):

def unique_randoms(n, max = nil)
 seen = {}
 n.times{x = rand max; seen[x] ? redo : seen[x] = 1}
 seen.keys
end

It's a little more terse than it needs to be. Here is a more explicit version:

def unique_randoms(n, max = nil)
 seen = {}
 n.times do
   x = rand max
   redo if seen[x]
   seen[x] = true
 end
 seen.keys
end

If you need a large number of randoms it is faster to use the Array sort method:

def unique_randoms(n, max)
  (0..max).sort{rand}[0..n]
end

This leads us to a solution to quickly getting random rows through ActiveRecord:

def random(scope, num_randoms)
  count = scope.count
  unique_randoms(num_randoms, count){|i| scope.first :offset => i}
end

The above solution is appropriate for large result sets where :order => 'RAND()' is inefficient, and for small quantities of random rows. Each random row adds a query to this solution.