Select Random Rows Using Ruby On Rails

Posted by jcnnghm Fri, 07 Aug 2009 00:59:00 GMT

Today I was looking for a way to randomly select rows from a database using Rails. I implemented the following message, which will randomly select a specified number of random rows, or as many rows as are available if the requested number is greater than the available number.

def find_random(random_count, options = {})
  if random_count > count(options)
    find_every(options).sort_by{rand}
  else
    find_every(options.merge({:offset => rand(count(options) - random_count + 1), :limit => random_count})).sort_by{rand}
  end
end

The code first attempts to determine if there are enough records to randomize. After that, it randomly selects an offset and returns the specified number of rows at that offset, after randomly sorting them. The benefit that this has over the other methods, is that it should scale well with very large data sets. The downside is, of course, that the are sequential from the given offset. In my situation, this works fine. Depending on what the data is used for, this may not work well for you. If that is the case, the approach I would take is to randomly generate a number of offsets, and select individual records at each offset. This should perform reasonably quickly as long as only a random items are selected.

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