It's easy.
require 'jbuilder'
data = { :a => 1, :b => 2 }
out = Jbuilder.encode do |json|
json.result data do |kv|
json.letter kv[0]
json.number kv[1]
end
end
puts out #=> {"result":[{"letter":"a","number":1},{"letter":"b","number":2}]}