main.coffee

_ = require("underscore")

Parse.Cloud.beforeSave "Post", (request, response) ->
  required_fields = [
    "title",
    "author",
    "body"
  ]

  errors = []

  _.each required_fields, (field) ->
    if !request.object.get(field)
      errors.push "#{field} can't be blank."

  if errors.length > 0
    response.error errors
  else
    response.success()

sinatra_example.rb

post "/posts" do
  begin
    Parse::Object.new("Post").tap do |post|
      post['title']  = params[:title]
      post['author'] = params[:author]
      post['body']   = params[:body]
    end.save
    return erb(:posts)
  rescue Parse::ParseProtocolError => e
    errors = JSON.parse(e.response['error'])
    # do something
  end
end

I think I understand the meaning of the universe.