Build mongo condition hash recursively given a set of ranges and values

gistfile1.rb

def build_agg_hash(condition_hash)
  return '' unless condition_hash && condition_hash.length > 0
  range, value = condition_hash.shift
  {'$cond' => {
    '$if' => {'$gte' => range.first, '$lt' => range.last},
    '$then' => value,
    '$else' => build_agg_hash(condition_hash)
  }}
end

build_agg_hash({
 (1..10) => 'one', 
 (11..20) => 'two', 
 (20..30) => 'three'
})