how to read a User uploaded file, without saving it to the database

You are very close. Check the class type of params[:uploaded_file], it should typically be either a StringIO or a Tempfile object -- both of which already act as files, and can be read using their respective read method(s).

Just to be sure (the class type of params[:uploaded_file] may vary depending on whether you are using Mongrel, Passenger, Webrick etc.) you can do a slightly more exhaustive attempt:

# Note: use form validation to ensure that
#  params[:uploaded_file] is not null

file_data = params[:uploaded_file]
if file_data.respond_to?(:read)
  xml_contents = file_data.read
elsif file_data.respond_to?(:path)
  xml_contents = File.read(file_data.path)
else
  logger.error "Bad file_data: #{file_data.class.name}: #{file_data.inspect}"
end

If, in your case, it turns out that params[:uploaded_file] is a hash, make sure that you have not mistakingly flipped the object_name and method parameters when invoking file_field in your view, or that your server is not giving you a hash with keys like :content_type etc. (in which case please comment on this post with the Bad file_data ... output from development.log/production.log.)

I need to read yaml files. I use remotipart and here the code:

in html.slim

 =form_tag('/locations/check_for_import', method: :post, remote: true, multipart: true)

...

<input id="uploadInput" type="file" name="uploadInput">

in controller

content = File.read(params[:uploadInput].tempfile)
doc = YAML.load(content)